A leap year (in the Gregorian calendar) occurs:
- In every year that is evenly divisible by 4.
- Unless the year is evenly divisible by 100, in which case it's only a leap year if the year is also evenly divisible by 400.
Some examples:
- 1997 was not a leap year as it's not divisible by 4.
- 1900 was not a leap year as it's not divisible by 400.
- 2000 was a leap year!
For a delightful, four-minute explanation of the whole phenomenon of leap years, check out [this YouTube video](https://www.youtube.com/watch?v=xX96xng7sAE).