Every four years in the Gregorian calendar, an extra day is added to the calendar in order to synchronize it with the solar year.
Occurs in every year that is divisible by four and only in century years that are evenly divided by 400. – Ex. 800, 1200, 2000 were leap years…BUT 1700 and 1900 were not because they are not divisible by 400, even though they are divisible by four.
In a leap year, the extra day is added at the end of February, giving it 29 days instead of 28.
The extra day is called a leap day, or an intercalary day.
It takes the earth a little bit less than 365 ¼ days (365.242 days) to orbit the Sun (solar year). For this reason, the full day is only added once every four years. In the Julian calendar, there is still an 11-minute, 14-second discrepancy each year.
46 B.C. – The practice of adding the extra day begins with the creation of the Julian calendar and a decree by Julius Caesar. The Julian calendar creates an extra day every four years, and does not follow the century-divisible-by-400 rule.
1582 A.D. – The 11-minute discrepancy in the Julian calendar adds up to ten days by this point. Pope Gregory XIII creates the Gregorian calendar and drops ten days from the month of October. He also establishes February 29th as the official date to add during a leap year, coins the term leap year, and creates the rules for adding the leap year.
Currently the solar year is approximately 26 seconds shorter than the Gregorian year.
Within a century, the fourth year is the first leap year.
In the U.S., leap year coincides with presidential election years.