Leap year
What is a leap year?
A year with 29 February. The Earth takes about 365.242 days to go around the Sun, so a 365-day calendar drifts. Adding a day most years that divide by 4 — but not most centuries — keeps the seasons from walking off the months. There is a leap-year lesson that runs this as a class.
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.