February 29

February 29 is known as a leap day (or "leap year day"), which is periodically added as the last day of the month to the Julian and Gregorian calendars, as an intercalary date, to create leap years. Its main function is to keep the calendars aligned with the Earth's seasons, as the planet orbits the sun.

In the older Julian calendar, leap days are added to every year that is evenly divisible by four. However, this causes the calendar to slowly become misaligned with the Earth's seasons, by about 3 days every 400 years. (The Julian calendar continues to be used in some places as the basis for their liturgical calendar).

The Gregorian calendar is the standard civil calendar used in most of the world, and is a slight modification of the Julian calendar. In this calendar, a centennial year does not have a February 29 (despite being evenly divisible by 4) unless it is also evenly divisible by 400. Thus, in contrast to the Julian calendar, the years 1700, 1800 and 1900 did not have leap year days but both calendars had a February 29 in the years 1600 and 2000.

The convention of adding a "February 29" to be the leap day was not widely used before the 15th century. Instead, beginning with Julius Caesar's calendar edict of 45 BC until the 16th century (formally), February 24 was doubled – two successive days had the same date.

February 29 is the 60th day of a leap year for both the Julian and Gregorian calendars, and 306 days remain until the end of the leap year. It is also the last day of meteorological winter in the Northern Hemisphere and the last day of meteorological summer in the Southern Hemisphere in leap years.