A calendar year has 365 days, but Earth takes about 365.2422 days to go around the Sun from one spring equinox to the next. Without a correction the calendar would drift by almost a day every four years — after a few centuries, the seasons would arrive weeks “late”. Leap years fix this by adding an extra day, February 29.
The Gregorian rule
- A year divisible by 4 is a leap year…
- …except a year divisible by 100, which is not…
- …unless it is also divisible by 400, which is.
So 2000 was a leap year, but 1900 was not, and 2100 won’t be. This gives an average year of 365.2425 days, close enough that the calendar drifts by only about one day in 3,000 years.
A little history
The Julian calendar, introduced by Julius Caesar in 46 BC, added a leap day every four years without exception. That overcorrected slightly, and by the 16th century the calendar was about ten days out of step with the equinoxes. Pope Gregory XIII introduced the current rule in 1582. Countries adopted it at different times; Britain and its colonies, including those in America, switched in 1752 and skipped eleven days that September.
The next leap years
The next leap years are 2028, 2032, 2036, 2040. 2026 is not a leap year, so February 2026 has 28 days.
Leap day facts
People born on February 29 — “leaplings” — usually celebrate on February 28 or March 1 in common years. A leap year also shifts the weekday of every date after February by two days instead of one, which is why the same calendar layout repeats in an irregular pattern of 6, 11 or 28 years. You can see it for yourself by comparing the February 2028 calendar with February 2026, or browse calendars for every year.