When did leap year begin?

In 46 B.C., Julius Caesar created leap years in the Julian calendar to accommodate his astronomer's estimate that one year was 365.25 days. By adding one day to the calendar every four years, the calendar would align with the seasons.

While the Julian calendar worked for a while, by the 16th century, the calendar and seasons were off by about 11 days. In March of 1582, Pope Gregory XII instituted the Gregorian calendar, moving the calendar forward by 11 days. The Gregorian calendar, still in use today, changed leap years too. While leap years occur every four years, years that mark the turn of the century, ending in 00, are not leap years unless they are also divisible by 400.