Why we have leap years

Why does February sometimes get an extra day? Our calendar needs a small adjustment to follow Earth’s year.

Earth takes about 365.2422 days to go around the Sun. That fraction matters when we count whole days on a calendar.

A regular calendar year has 365 days. So the calendar drifts by almost a quarter day each year. The difference keeps building up.

Adding an extra day every four years fixes most of that drift. The extra day is February 29.

But this adds a little too much time. The calendar needs an occasional exception to keep that overcorrection small.

The Gregorian calendar, introduced in 1582, makes that adjustment. It skips leap days in century years, unless the year divides evenly by 400.

That rule explains why 1900 was not a leap year. But 2000 divided evenly by 400, so it kept its leap day.

Leap years keep calendar dates close to Earth’s yearly cycle through the seasons.