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 2008, 2012, 2016, 2020, and 2024. Years that are divisible by 100, but not by 400, do not contain a leap day. Thus, 1700, 1800, and 1900 did not contain a leap day, 2100, 2200, and 2300 will not contain a leap day, while 1600 and 2000 did, and 2400 will. 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. In theChinese calendar, this day will only occur in years of the monkey, dragon, and rat.
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, in future years theseasons would occur later 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 completes its orbit around the Sun in 365 days, 5 hours, 49 minutes, and 16 seconds (365.2425 days). On these figures adding a calendar day every four years is an excess of 42 minutes and 52 seconds each time, or about 3 days every 400 years. To compensate for this, three days are removed every 400 years.
The Gregorian calendar reform implements 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 is not a leap year unless that year was also exactly divisible by 400. This means that the years 1600, 2000, and 2400 are leap years, while the years 1700, 1800, 1900, 2100, 2200, 2300, and 2500 are not leap years.
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 order of the leap days is: Thursday, Tuesday, Sunday, Friday, Wednesday, Monday, and Saturday.