From 7fff1e8ef52d8a38f8297c0314115414b45643ea Mon Sep 17 00:00:00 2001
From: Marco Gorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Mon, 8 Apr 2024 18:44:54 +0100
Subject: [PATCH 1/4] feat: support weekend argument in business_day_count

---
 crates/polars-ops/src/series/ops/business.rs  | 14 ++--
 .../src/dsl/function_expr/business.rs         | 12 ++--
 .../polars-plan/src/dsl/functions/business.rs |  4 +-
 py-polars/polars/functions/business.py        | 65 +++++++++++++++++--
 py-polars/polars/type_aliases.py              |  9 +++
 py-polars/src/functions/business.rs           |  4 +-
 .../time_series/test_business_day_count.py    | 16 ++++-
 .../business/test_business_day_count.py       | 36 ++++++++++
 8 files changed, 138 insertions(+), 22 deletions(-)

diff --git a/crates/polars-ops/src/series/ops/business.rs b/crates/polars-ops/src/series/ops/business.rs
index 5b792453b5c4..3b975ec778ec 100644
--- a/crates/polars-ops/src/series/ops/business.rs
+++ b/crates/polars-ops/src/series/ops/business.rs
@@ -2,12 +2,18 @@ use polars_core::prelude::arity::binary_elementwise_values;
 use polars_core::prelude::*;
 
 /// Count the number of business days between `start` and `end`, excluding `end`.
-pub fn business_day_count(start: &Series, end: &Series) -> PolarsResult<Series> {
+///
+/// # Arguments
+/// - `start`: Series holding start dates.
+/// - `end`: Series holding end dates.
+/// - `week_mask`: A boolean array of length 7, where `true` indicates that the day is a business day.
+pub fn business_day_count(
+    start: &Series,
+    end: &Series,
+    week_mask: [bool; 7],
+) -> PolarsResult<Series> {
     let start_dates = start.date()?;
     let end_dates = end.date()?;
-
-    // TODO: support customising weekdays
-    let week_mask: [bool; 7] = [true, true, true, true, true, false, false];
     let n_business_days_in_week_mask = week_mask.iter().filter(|&x| *x).count() as i32;
 
     let out = match (start_dates.len(), end_dates.len()) {
diff --git a/crates/polars-plan/src/dsl/function_expr/business.rs b/crates/polars-plan/src/dsl/function_expr/business.rs
index f9a38b1165cc..745dcfdff8f5 100644
--- a/crates/polars-plan/src/dsl/function_expr/business.rs
+++ b/crates/polars-plan/src/dsl/function_expr/business.rs
@@ -12,7 +12,7 @@ use crate::prelude::SeriesUdf;
 #[derive(Clone, PartialEq, Debug, Eq, Hash)]
 pub enum BusinessFunction {
     #[cfg(feature = "business")]
-    BusinessDayCount,
+    BusinessDayCount { week_mask: [bool; 7] },
 }
 
 impl Display for BusinessFunction {
@@ -20,7 +20,7 @@ impl Display for BusinessFunction {
         use BusinessFunction::*;
         let s = match self {
             #[cfg(feature = "business")]
-            &BusinessDayCount => "business_day_count",
+            &BusinessDayCount { .. } => "business_day_count",
         };
         write!(f, "{s}")
     }
@@ -30,16 +30,16 @@ impl From<BusinessFunction> for SpecialEq<Arc<dyn SeriesUdf>> {
         use BusinessFunction::*;
         match func {
             #[cfg(feature = "business")]
-            BusinessDayCount => {
-                map_as_slice!(business_day_count)
+            BusinessDayCount { week_mask } => {
+                map_as_slice!(business_day_count, week_mask)
             },
         }
     }
 }
 
 #[cfg(feature = "business")]
-pub(super) fn business_day_count(s: &[Series]) -> PolarsResult<Series> {
+pub(super) fn business_day_count(s: &[Series], week_mask: [bool; 7]) -> PolarsResult<Series> {
     let start = &s[0];
     let end = &s[1];
-    polars_ops::prelude::business_day_count(start, end)
+    polars_ops::prelude::business_day_count(start, end, week_mask)
 }
diff --git a/crates/polars-plan/src/dsl/functions/business.rs b/crates/polars-plan/src/dsl/functions/business.rs
index 4bfdcc0b20cc..0a0210ced57f 100644
--- a/crates/polars-plan/src/dsl/functions/business.rs
+++ b/crates/polars-plan/src/dsl/functions/business.rs
@@ -1,12 +1,12 @@
 use super::*;
 
 #[cfg(feature = "dtype-date")]
-pub fn business_day_count(start: Expr, end: Expr) -> Expr {
+pub fn business_day_count(start: Expr, end: Expr, week_mask: [bool; 7]) -> Expr {
     let input = vec![start, end];
 
     Expr::Function {
         input,
-        function: FunctionExpr::Business(BusinessFunction::BusinessDayCount {}),
+        function: FunctionExpr::Business(BusinessFunction::BusinessDayCount { week_mask }),
         options: FunctionOptions {
             allow_rename: true,
             ..Default::default()
diff --git a/py-polars/polars/functions/business.py b/py-polars/polars/functions/business.py
index ae5791fde2a6..f65292f10ef4 100644
--- a/py-polars/polars/functions/business.py
+++ b/py-polars/polars/functions/business.py
@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 import contextlib
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Iterable
 
 from polars._utils.parse_expr_input import parse_as_expression
 from polars._utils.wrap import wrap_expr
@@ -13,25 +13,61 @@
     from datetime import date
 
     from polars import Expr
-    from polars.type_aliases import IntoExprColumn
+    from polars.type_aliases import DayOfWeek, IntoExprColumn
+
+DAY_NAMES = (
+    "Mon",
+    "Tue",
+    "Wed",
+    "Thu",
+    "Fri",
+    "Sat",
+    "Sun",
+)
+
+
+def _make_week_mask(
+    weekend: Iterable[str] | None,
+) -> tuple[bool, ...]:
+    if weekend is None:
+        return tuple([True] * 7)
+    if isinstance(weekend, str):
+        weekend_set = {weekend}
+    else:
+        weekend_set = set(weekend)
+    for day in weekend_set:
+        if day not in DAY_NAMES:
+            msg = f"Expected one of {DAY_NAMES}, got: {day}"
+            raise ValueError(msg)
+    return tuple(
+        [
+            False if v in weekend else True  # noqa: SIM211
+            for v in DAY_NAMES
+        ]
+    )
 
 
 def business_day_count(
     start: date | IntoExprColumn,
     end: date | IntoExprColumn,
+    weekend: DayOfWeek | Iterable[DayOfWeek] | None = ("Sat", "Sun"),
 ) -> Expr:
     """
     Count the number of business days between `start` and `end` (not including `end`).
 
-    By default, Saturday and Sunday are excluded. The ability to
-    customise week mask and holidays is not yet implemented.
-
     Parameters
     ----------
     start
         Start dates.
     end
         End dates.
+    weekend
+        Which days of the week to exclude. The default is `('Sat', 'Sun')`, but you
+        can also pass, for example, `weekend=('Fri', 'Sat')`, `weekend='Sun'`,
+        or `weekend=None`.
+
+        Allowed values in the tuple are 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat',
+        and 'Sun'.
 
     Returns
     -------
@@ -62,7 +98,24 @@ def business_day_count(
 
     Note how the two "count" columns differ due to the weekend (2020-01-04 - 2020-01-05)
     not being counted by `business_day_count`.
+
+    You can pass a custom weekend - for example, if you only take Sunday off:
+
+    >>> df.with_columns(
+    ...     total_day_count=(pl.col("end") - pl.col("start")).dt.total_days(),
+    ...     business_day_count=pl.business_day_count("start", "end", weekend="Sun"),
+    ... )
+    shape: (2, 4)
+    ┌────────────┬────────────┬─────────────────┬────────────────────┐
+    │ start      ┆ end        ┆ total_day_count ┆ business_day_count │
+    │ ---        ┆ ---        ┆ ---             ┆ ---                │
+    │ date       ┆ date       ┆ i64             ┆ i32                │
+    ╞════════════╪════════════╪═════════════════╪════════════════════╡
+    │ 2020-01-01 ┆ 2020-01-02 ┆ 1               ┆ 1                  │
+    │ 2020-01-02 ┆ 2020-01-10 ┆ 8               ┆ 7                  │
+    └────────────┴────────────┴─────────────────┴────────────────────┘
     """
     start_pyexpr = parse_as_expression(start)
     end_pyexpr = parse_as_expression(end)
-    return wrap_expr(plr.business_day_count(start_pyexpr, end_pyexpr))
+    week_mask = _make_week_mask(weekend)
+    return wrap_expr(plr.business_day_count(start_pyexpr, end_pyexpr, week_mask))
diff --git a/py-polars/polars/type_aliases.py b/py-polars/polars/type_aliases.py
index 2443ce4f64ad..bdc9e741e71b 100644
--- a/py-polars/polars/type_aliases.py
+++ b/py-polars/polars/type_aliases.py
@@ -158,6 +158,15 @@
     "horizontal",
     "align",
 ]
+DayOfWeek = Literal[
+    "Mon",
+    "Tue",
+    "Wed",
+    "Thu",
+    "Fri",
+    "Sat",
+    "Sun",
+]
 EpochTimeUnit = Literal["ns", "us", "ms", "s", "d"]
 Orientation: TypeAlias = Literal["col", "row"]
 SearchSortedSide: TypeAlias = Literal["any", "left", "right"]
diff --git a/py-polars/src/functions/business.rs b/py-polars/src/functions/business.rs
index 246f902b895a..0ca6ec058d4a 100644
--- a/py-polars/src/functions/business.rs
+++ b/py-polars/src/functions/business.rs
@@ -4,8 +4,8 @@ use pyo3::prelude::*;
 use crate::PyExpr;
 
 #[pyfunction]
-pub fn business_day_count(start: PyExpr, end: PyExpr) -> PyExpr {
+pub fn business_day_count(start: PyExpr, end: PyExpr, week_mask: [bool; 7]) -> PyExpr {
     let start = start.inner;
     let end = end.inner;
-    dsl::business_day_count(start, end).into()
+    dsl::business_day_count(start, end, week_mask).into()
 }
diff --git a/py-polars/tests/parametric/time_series/test_business_day_count.py b/py-polars/tests/parametric/time_series/test_business_day_count.py
index 0cb1bf95df33..04a2553e7aaf 100644
--- a/py-polars/tests/parametric/time_series/test_business_day_count.py
+++ b/py-polars/tests/parametric/time_series/test_business_day_count.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import datetime as dt
+from typing import TYPE_CHECKING
 
 import hypothesis.strategies as st
 import numpy as np
@@ -8,22 +9,33 @@
 
 import polars as pl
 from polars._utils.various import parse_version
+from polars.functions.business import _make_week_mask
+
+if TYPE_CHECKING:
+    from polars.type_aliases import DayOfWeek
 
 
 @given(
     start=st.dates(min_value=dt.date(1969, 1, 1), max_value=dt.date(1970, 12, 31)),
     end=st.dates(min_value=dt.date(1969, 1, 1), max_value=dt.date(1970, 12, 31)),
+    weekend=st.lists(
+        st.sampled_from(["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]),
+        min_size=0,
+        max_size=6,
+        unique=True,
+    ),
 )
 def test_against_np_busday_count(
     start: dt.date,
     end: dt.date,
+    weekend: list[DayOfWeek],
 ) -> None:
     result = (
         pl.DataFrame({"start": [start], "end": [end]})
-        .select(n=pl.business_day_count("start", "end"))["n"]
+        .select(n=pl.business_day_count("start", "end", weekend=weekend))["n"]
         .item()
     )
-    expected = np.busday_count(start, end)
+    expected = np.busday_count(start, end, weekmask=_make_week_mask(weekend))
     if start > end and parse_version(np.__version__) < parse_version("1.25"):
         # Bug in old versions of numpy
         reject()
diff --git a/py-polars/tests/unit/functions/business/test_business_day_count.py b/py-polars/tests/unit/functions/business/test_business_day_count.py
index 74befbd3268b..6feced25e2bb 100644
--- a/py-polars/tests/unit/functions/business/test_business_day_count.py
+++ b/py-polars/tests/unit/functions/business/test_business_day_count.py
@@ -1,5 +1,7 @@
 from datetime import date
 
+import pytest
+
 import polars as pl
 from polars.testing import assert_series_equal
 
@@ -50,6 +52,40 @@ def test_business_day_count() -> None:
     assert_series_equal(result, expected)
 
 
+def test_business_day_count_w_weekend() -> None:
+    df = pl.DataFrame(
+        {
+            "start": [date(2020, 1, 1), date(2020, 1, 2)],
+            "end": [date(2020, 1, 2), date(2020, 1, 10)],
+        }
+    )
+    result = df.select(
+        business_day_count=pl.business_day_count("start", "end", weekend="Sun"),
+    )["business_day_count"]
+    expected = pl.Series("business_day_count", [1, 7], pl.Int32)
+    assert_series_equal(result, expected)
+    result = df.select(
+        business_day_count=pl.business_day_count(
+            "start", "end", weekend=("Thu", "Fri", "Sat")
+        ),
+    )["business_day_count"]
+    expected = pl.Series("business_day_count", [1, 4], pl.Int32)
+    assert_series_equal(result, expected)
+    result = df.select(
+        business_day_count=pl.business_day_count("start", "end", weekend=None),
+    )["business_day_count"]
+    expected = pl.Series("business_day_count", [1, 8], pl.Int32)
+    assert_series_equal(result, expected)
+
+
+def test_business_day_count_w_weekend_invalid() -> None:
+    msg = r"Expected one of \('Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun'\), got: cabbage"
+    with pytest.raises(ValueError, match=msg):
+        pl.business_day_count("start", "end", weekend="cabbage")  # type: ignore[arg-type]
+    with pytest.raises(ValueError, match=msg):
+        pl.business_day_count("start", "end", weekend=("Sat", "cabbage"))  # type: ignore[arg-type]
+
+
 def test_business_day_count_schema() -> None:
     lf = pl.LazyFrame(
         {

From 81cd34a2760f47612ac40ed790bc9ac94e673dcb Mon Sep 17 00:00:00 2001
From: Marco Gorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Tue, 9 Apr 2024 08:47:41 +0100
Subject: [PATCH 2/4] cache day names

---
 py-polars/polars/functions/business.py | 28 +++++++++++++++-----------
 1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/py-polars/polars/functions/business.py b/py-polars/polars/functions/business.py
index f65292f10ef4..c4d8d73ab24d 100644
--- a/py-polars/polars/functions/business.py
+++ b/py-polars/polars/functions/business.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import contextlib
+import functools
 from typing import TYPE_CHECKING, Iterable
 
 from polars._utils.parse_expr_input import parse_as_expression
@@ -15,15 +16,18 @@
     from polars import Expr
     from polars.type_aliases import DayOfWeek, IntoExprColumn
 
-DAY_NAMES = (
-    "Mon",
-    "Tue",
-    "Wed",
-    "Thu",
-    "Fri",
-    "Sat",
-    "Sun",
-)
+
+@functools.lru_cache
+def _day_names() -> tuple[str, ...]:
+    return (
+        "Mon",
+        "Tue",
+        "Wed",
+        "Thu",
+        "Fri",
+        "Sat",
+        "Sun",
+    )
 
 
 def _make_week_mask(
@@ -36,13 +40,13 @@ def _make_week_mask(
     else:
         weekend_set = set(weekend)
     for day in weekend_set:
-        if day not in DAY_NAMES:
-            msg = f"Expected one of {DAY_NAMES}, got: {day}"
+        if day not in _day_names():
+            msg = f"Expected one of {_day_names()}, got: {day}"
             raise ValueError(msg)
     return tuple(
         [
             False if v in weekend else True  # noqa: SIM211
-            for v in DAY_NAMES
+            for v in _day_names()
         ]
     )
 

From fe61fc28fd06a379fc7cacd4e37fdefde52f0032 Mon Sep 17 00:00:00 2001
From: Marco Gorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Tue, 9 Apr 2024 09:12:37 +0100
Subject: [PATCH 3/4] extra test

---
 py-polars/polars/functions/business.py                     | 6 +++---
 .../unit/functions/business/test_business_day_count.py     | 7 +++++++
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/py-polars/polars/functions/business.py b/py-polars/polars/functions/business.py
index c4d8d73ab24d..df23d9700eb6 100644
--- a/py-polars/polars/functions/business.py
+++ b/py-polars/polars/functions/business.py
@@ -36,10 +36,10 @@ def _make_week_mask(
     if weekend is None:
         return tuple([True] * 7)
     if isinstance(weekend, str):
-        weekend_set = {weekend}
+        weekend = {weekend}
     else:
-        weekend_set = set(weekend)
-    for day in weekend_set:
+        weekend = set(weekend)
+    for day in weekend:
         if day not in _day_names():
             msg = f"Expected one of {_day_names()}, got: {day}"
             raise ValueError(msg)
diff --git a/py-polars/tests/unit/functions/business/test_business_day_count.py b/py-polars/tests/unit/functions/business/test_business_day_count.py
index 6feced25e2bb..bf08f4ac98fb 100644
--- a/py-polars/tests/unit/functions/business/test_business_day_count.py
+++ b/py-polars/tests/unit/functions/business/test_business_day_count.py
@@ -64,6 +64,13 @@ def test_business_day_count_w_weekend() -> None:
     )["business_day_count"]
     expected = pl.Series("business_day_count", [1, 7], pl.Int32)
     assert_series_equal(result, expected)
+
+    result = df.select(
+        business_day_count=pl.business_day_count("start", "end", weekend=("Sun",)),
+    )["business_day_count"]
+    expected = pl.Series("business_day_count", [1, 7], pl.Int32)
+    assert_series_equal(result, expected)
+
     result = df.select(
         business_day_count=pl.business_day_count(
             "start", "end", weekend=("Thu", "Fri", "Sat")

From 763537920327166c0aef230ed99b02eecce0ecc7 Mon Sep 17 00:00:00 2001
From: Marco Gorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Tue, 9 Apr 2024 20:48:44 +0100
Subject: [PATCH 4/4] keep it simple

---
 crates/polars-ops/src/series/ops/business.rs  |  3 ++
 py-polars/polars/functions/business.py        | 54 +++----------------
 py-polars/polars/type_aliases.py              |  9 ----
 .../time_series/test_business_day_count.py    | 23 ++++----
 .../business/test_business_day_count.py       | 38 +++++++------
 5 files changed, 38 insertions(+), 89 deletions(-)

diff --git a/crates/polars-ops/src/series/ops/business.rs b/crates/polars-ops/src/series/ops/business.rs
index 3b975ec778ec..115ccf8ae389 100644
--- a/crates/polars-ops/src/series/ops/business.rs
+++ b/crates/polars-ops/src/series/ops/business.rs
@@ -12,6 +12,9 @@ pub fn business_day_count(
     end: &Series,
     week_mask: [bool; 7],
 ) -> PolarsResult<Series> {
+    if !week_mask.iter().any(|&x| x) {
+        polars_bail!(ComputeError:"`week_mask` must have at least one business day");
+    }
     let start_dates = start.date()?;
     let end_dates = end.date()?;
     let n_business_days_in_week_mask = week_mask.iter().filter(|&x| *x).count() as i32;
diff --git a/py-polars/polars/functions/business.py b/py-polars/polars/functions/business.py
index df23d9700eb6..125bda15113e 100644
--- a/py-polars/polars/functions/business.py
+++ b/py-polars/polars/functions/business.py
@@ -1,7 +1,6 @@
 from __future__ import annotations
 
 import contextlib
-import functools
 from typing import TYPE_CHECKING, Iterable
 
 from polars._utils.parse_expr_input import parse_as_expression
@@ -14,47 +13,13 @@
     from datetime import date
 
     from polars import Expr
-    from polars.type_aliases import DayOfWeek, IntoExprColumn
-
-
-@functools.lru_cache
-def _day_names() -> tuple[str, ...]:
-    return (
-        "Mon",
-        "Tue",
-        "Wed",
-        "Thu",
-        "Fri",
-        "Sat",
-        "Sun",
-    )
-
-
-def _make_week_mask(
-    weekend: Iterable[str] | None,
-) -> tuple[bool, ...]:
-    if weekend is None:
-        return tuple([True] * 7)
-    if isinstance(weekend, str):
-        weekend = {weekend}
-    else:
-        weekend = set(weekend)
-    for day in weekend:
-        if day not in _day_names():
-            msg = f"Expected one of {_day_names()}, got: {day}"
-            raise ValueError(msg)
-    return tuple(
-        [
-            False if v in weekend else True  # noqa: SIM211
-            for v in _day_names()
-        ]
-    )
+    from polars.type_aliases import IntoExprColumn
 
 
 def business_day_count(
     start: date | IntoExprColumn,
     end: date | IntoExprColumn,
-    weekend: DayOfWeek | Iterable[DayOfWeek] | None = ("Sat", "Sun"),
+    week_mask: Iterable[bool] = (True, True, True, True, True, False, False),
 ) -> Expr:
     """
     Count the number of business days between `start` and `end` (not including `end`).
@@ -65,13 +30,10 @@ def business_day_count(
         Start dates.
     end
         End dates.
-    weekend
-        Which days of the week to exclude. The default is `('Sat', 'Sun')`, but you
-        can also pass, for example, `weekend=('Fri', 'Sat')`, `weekend='Sun'`,
-        or `weekend=None`.
-
-        Allowed values in the tuple are 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat',
-        and 'Sun'.
+    week_mask
+        Which days of the week to count. The default is Monday to Friday.
+        If you wanted to count only Monday to Thursday, you would pass
+        `(True, True, True, True, False, False, False)`.
 
     Returns
     -------
@@ -105,9 +67,10 @@ def business_day_count(
 
     You can pass a custom weekend - for example, if you only take Sunday off:
 
+    >>> week_mask = (True, True, True, True, True, True, False)
     >>> df.with_columns(
     ...     total_day_count=(pl.col("end") - pl.col("start")).dt.total_days(),
-    ...     business_day_count=pl.business_day_count("start", "end", weekend="Sun"),
+    ...     business_day_count=pl.business_day_count("start", "end", week_mask),
     ... )
     shape: (2, 4)
     ┌────────────┬────────────┬─────────────────┬────────────────────┐
@@ -121,5 +84,4 @@ def business_day_count(
     """
     start_pyexpr = parse_as_expression(start)
     end_pyexpr = parse_as_expression(end)
-    week_mask = _make_week_mask(weekend)
     return wrap_expr(plr.business_day_count(start_pyexpr, end_pyexpr, week_mask))
diff --git a/py-polars/polars/type_aliases.py b/py-polars/polars/type_aliases.py
index bdc9e741e71b..2443ce4f64ad 100644
--- a/py-polars/polars/type_aliases.py
+++ b/py-polars/polars/type_aliases.py
@@ -158,15 +158,6 @@
     "horizontal",
     "align",
 ]
-DayOfWeek = Literal[
-    "Mon",
-    "Tue",
-    "Wed",
-    "Thu",
-    "Fri",
-    "Sat",
-    "Sun",
-]
 EpochTimeUnit = Literal["ns", "us", "ms", "s", "d"]
 Orientation: TypeAlias = Literal["col", "row"]
 SearchSortedSide: TypeAlias = Literal["any", "left", "right"]
diff --git a/py-polars/tests/parametric/time_series/test_business_day_count.py b/py-polars/tests/parametric/time_series/test_business_day_count.py
index 04a2553e7aaf..7d9d61fbc5cb 100644
--- a/py-polars/tests/parametric/time_series/test_business_day_count.py
+++ b/py-polars/tests/parametric/time_series/test_business_day_count.py
@@ -1,41 +1,36 @@
 from __future__ import annotations
 
 import datetime as dt
-from typing import TYPE_CHECKING
 
 import hypothesis.strategies as st
 import numpy as np
-from hypothesis import given, reject
+from hypothesis import assume, given, reject
 
 import polars as pl
 from polars._utils.various import parse_version
-from polars.functions.business import _make_week_mask
-
-if TYPE_CHECKING:
-    from polars.type_aliases import DayOfWeek
 
 
 @given(
     start=st.dates(min_value=dt.date(1969, 1, 1), max_value=dt.date(1970, 12, 31)),
     end=st.dates(min_value=dt.date(1969, 1, 1), max_value=dt.date(1970, 12, 31)),
-    weekend=st.lists(
-        st.sampled_from(["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]),
-        min_size=0,
-        max_size=6,
-        unique=True,
+    week_mask=st.lists(
+        st.sampled_from([True, False]),
+        min_size=7,
+        max_size=7,
     ),
 )
 def test_against_np_busday_count(
     start: dt.date,
     end: dt.date,
-    weekend: list[DayOfWeek],
+    week_mask: tuple[bool, ...],
 ) -> None:
+    assume(any(week_mask))
     result = (
         pl.DataFrame({"start": [start], "end": [end]})
-        .select(n=pl.business_day_count("start", "end", weekend=weekend))["n"]
+        .select(n=pl.business_day_count("start", "end", week_mask=week_mask))["n"]
         .item()
     )
-    expected = np.busday_count(start, end, weekmask=_make_week_mask(weekend))
+    expected = np.busday_count(start, end, weekmask=week_mask)
     if start > end and parse_version(np.__version__) < parse_version("1.25"):
         # Bug in old versions of numpy
         reject()
diff --git a/py-polars/tests/unit/functions/business/test_business_day_count.py b/py-polars/tests/unit/functions/business/test_business_day_count.py
index bf08f4ac98fb..13a1a05dbb7b 100644
--- a/py-polars/tests/unit/functions/business/test_business_day_count.py
+++ b/py-polars/tests/unit/functions/business/test_business_day_count.py
@@ -52,7 +52,7 @@ def test_business_day_count() -> None:
     assert_series_equal(result, expected)
 
 
-def test_business_day_count_w_weekend() -> None:
+def test_business_day_count_w_week_mask() -> None:
     df = pl.DataFrame(
         {
             "start": [date(2020, 1, 1), date(2020, 1, 2)],
@@ -60,37 +60,35 @@ def test_business_day_count_w_weekend() -> None:
         }
     )
     result = df.select(
-        business_day_count=pl.business_day_count("start", "end", weekend="Sun"),
-    )["business_day_count"]
-    expected = pl.Series("business_day_count", [1, 7], pl.Int32)
-    assert_series_equal(result, expected)
-
-    result = df.select(
-        business_day_count=pl.business_day_count("start", "end", weekend=("Sun",)),
+        business_day_count=pl.business_day_count(
+            "start", "end", week_mask=(True, True, True, True, True, True, False)
+        ),
     )["business_day_count"]
     expected = pl.Series("business_day_count", [1, 7], pl.Int32)
     assert_series_equal(result, expected)
 
     result = df.select(
         business_day_count=pl.business_day_count(
-            "start", "end", weekend=("Thu", "Fri", "Sat")
+            "start", "end", week_mask=(True, True, True, False, False, False, True)
         ),
     )["business_day_count"]
     expected = pl.Series("business_day_count", [1, 4], pl.Int32)
     assert_series_equal(result, expected)
-    result = df.select(
-        business_day_count=pl.business_day_count("start", "end", weekend=None),
-    )["business_day_count"]
-    expected = pl.Series("business_day_count", [1, 8], pl.Int32)
-    assert_series_equal(result, expected)
 
 
-def test_business_day_count_w_weekend_invalid() -> None:
-    msg = r"Expected one of \('Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun'\), got: cabbage"
-    with pytest.raises(ValueError, match=msg):
-        pl.business_day_count("start", "end", weekend="cabbage")  # type: ignore[arg-type]
-    with pytest.raises(ValueError, match=msg):
-        pl.business_day_count("start", "end", weekend=("Sat", "cabbage"))  # type: ignore[arg-type]
+def test_business_day_count_w_week_mask_invalid() -> None:
+    with pytest.raises(ValueError, match=r"expected a sequence of length 7 \(got 2\)"):
+        pl.business_day_count("start", "end", week_mask=(False, 0))  # type: ignore[arg-type]
+    df = pl.DataFrame(
+        {
+            "start": [date(2020, 1, 1), date(2020, 1, 2)],
+            "end": [date(2020, 1, 2), date(2020, 1, 10)],
+        }
+    )
+    with pytest.raises(
+        pl.ComputeError, match="`week_mask` must have at least one business day"
+    ):
+        df.select(pl.business_day_count("start", "end", week_mask=[False] * 7))
 
 
 def test_business_day_count_schema() -> None: