Leap year

Definition
A leap year is a year containing one additional day to keep the calendar year synchronized with the astronomical year. Because astronomical events do not repeat in a whole number of days, calendars that have the same number of days in each year, over time, will drift with respect to the event that the year is supposed to track. By inserting an additional day into the year, the drift can be corrected.

Since the earth's rotation around the sun takes slightly less than 365.25 days, years ending in 00 are not leap years except for years also divisible by 400.