<P> February 29, also known as leap day or leap year day, is a date added to most years that are divisible by 4, such as 2008, 2012, 2016, 2020, and 2024 . A leap day is added in various solar calendars (calendars based on the Earth's revolution around the Sun), including the Gregorian calendar standard in most of the world . Lunisolar calendars (whose months are based on the phases of the Moon) instead add a leap or intercalary month . </P> <P> In the Gregorian calendar, 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; neither will 2100, 2200, and 2300 . Conversely, 1600 and 2000 did and 2400 will . Years containing a leap day are called leap years . Years not containing a leap day are called common 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 the Chinese calendar, this day will only occur in years of the monkey, dragon, and rat . </P> <P> A leap day is observed because a complete revolution around the Sun takes approximately 6 hours longer than 365 days (8,760 hours). A leap day compensates for this lag, realigning the calendar with the Earth's position in the Solar System; otherwise, seasons would occur earlier than intended in the calendar year . The Julian calendar used in Christendom until the 16th century added a leap day every four years; but this rule adds too many days (roughly 3 every 400 years), making the equinoxes and solstices shift gradually to earlier dates . By the 16th century the vernal equinox had drifted to March 11, and the Gregorian calendar was introduced both to shift it back by omitting several days, and to reduce the number of leap years via the "century rule" to keep the equinoxes more or less fixed and the date of Easter consistently close to the vernal equinox . </P> <P> 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 the seasons would occur later in the calendar, eventually leading to confusion about when to undertake activities dependent on weather, ecology, or hours of daylight . </P>

Why there are 29 days in february in a leap year