Leap year
What is a leap year?
A leap year has an extra day, February 29, giving it 366 days. That extra day helps keep our calendar aligned with the seasons because a year is a little longer than 365 days.
The leftover quarter-day
If every year were 365 days, the calendar would lose about a quarter-day a year — a day every four years, a month every 120, a season every 700. Spring would slide into December.
Most years that divide by 4 get 29 February. 1900 did not. 2000 did. That is the century rule, and it is why the calendar still matches the seasons.
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The year is not 365.25 either
365.2422, more nearly. The Gregorian skip-three-centuries-in-four rule approximates that. The remaining error is about a day in a few thousand years. Good enough that the seasons stay put on the months we named for them.