A leap year is a calendar year during which 1 extra day is added in order to keep the calendar year synchronized with whatever astronomical or seasonal event the calendar is based on.
In the case of the Gregorian calendar, the calendar used throughout most of the world, a calendar year is based on the Earth's revolution around the Sun. A year in the Gregorian calendar is referred to as a tropical year, and there are an average of 365.2422 days in a tropical year. The Gregorian calendar uses a combination of common years and leap years to maintain this average.
|Days in a common year||:||365|
|Days in a leap year||:||366|
The rule used to determine whether a year is a common year or a leap year, as described by the United States Naval Observatory, is as follows:
|"Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the years 1600 and 2000 are."|
Leap years occur in February; while February typically has 28 days, during a leap year, it has 29 days. Below is a table that shows which years between 2000 and 2080 are leap years.