February 29, also known as the leap day of the Gregorian calendar, is a date that occurs in most years that are divisible by 4, such as 2004, 2008, 2012, 2016 and 2020. Years that are divisible by 100, but not by 400, do not contain a leap day; thus 1900 did not contain a leap day while 2000 did. Years containing a leap day are called leap years. February 29 is the 60th day of the Gregorian calendar in such a year, with 306 days remaining until the end of the year.
Although most modern calendar years have 365 days, a complete revolution around the sun (one Solar year) takes approximately 365 days and 6 hours. An extra 24 hours thus accumulates every four years, requiring that an extra calendar day be added to align the calendar with the sun’s apparent position. Without the added day, the seasons would move back in the calendar, leading to confusion about when to undertake activities dependent on weather, ecology, or hours of daylight.
A solar year, however, is slightly shorter than 365 days and 6 hours (365.25 days). More precisely, as derived from the Alfonsine tables, the Earth makes a complete revolution around the sun in 365 days, 5 hours, 49 minutes, and 16 seconds (365.2425 days). Adding a calendar day every four years would thus add an extra 43 minutes and 12 seconds to the calendar, or 3 days every 400 years. To compensate for this, three leap days must be omitted every 400 years. The Gregorian calendar reform implemented this adjustment by making an exception to the general rule that there is a leap year every four years. Instead, a year divisible by 100 would not be a leap year unless that year was also exactly divisible by 400. This means that the years 1600 and 2000 were leap years, as will be 2400 and 2800, but the years 1700, 1800, and 1900 were not, nor will 2100, 2200, and 2300.
The Gregorian calendar repeats itself every 400 years, which is exactly 20,871 weeks including 97 leap days. Over this period, February 29 falls on Sunday, Tuesday, and Thursday 13 times each; 14 times each on Friday and Saturday; and 15 times each on Monday and Wednesday.
The concepts of the leap year and leap day are distinct from the leap second, which results from changes in the Earth’s rotational speed.
The leap day was introduced as part of the Julian reform. The day following the Terminalia (February 23) was doubled, forming the “bis sextum”—literally ‘double sixth’, since February 24 was ‘the sixth day before the Kalends of March’ using Roman inclusive counting (March 1 was the ‘first day’). Although exceptions exist, the first day of the bis sextum (February 24) was usually regarded as the intercalated or “bissextile” day since the third century. February 29 came to be regarded as the leap day when the Roman system of numbering days was replaced by sequential numbering in the late Middle Ages.