Leap Year

A calendar year containing one extra day — February 29 in the Gregorian calendar — added approximately every four years to keep the calendar year aligned with the astronomical solar year of 365.2422 days. The Gregorian reform refined the rule so that century years are leap years only when divisible by 400, meaning 1900 was not a leap year but 2000 was, and leap years subtly affect holiday scheduling because annual fixed-date holidays shift one extra weekday compared with regular years.