"And for each employee we want to find the first party date that comes after (or on) their birthday. We can express that with a rolling join:"

employees |> 
  left_join(parties, join_by(closest(birthday >= party)))

The sentence should rather be:
"And for each employee we want to find the last party date that comes before (or on) their birthday. We can express that with a rolling join:"

Alternatively, the code could be updated as follows

employees |> 
  left_join(parties, join_by(closest(birthday <= party)))

if we want parties after birthdays as mentioned in the original sentence