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, 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 the Chinese calendar, this day will only occur in years of the monkey, dragon, and rat.
A leap day is observed because a complete revolution around the Sun takes approximately 6 hours longer than 365 days (8,760 hours). It compensates for this lag, realigning the calendar with the Earth's position in the Solar System; otherwise, seasons would occur in a different time than intended in the calendar 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, 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.
A solar year is actually slightly shorter than 365 days and 6 hours (365.25 days). As early as the 13th century it was recognized that the year is shorter than the 365.25 days assumed by the Julian calendar: the Earth's orbital period around the Sun was derived from the medieval Alfonsine tables as 365 days, 5 hours, 49 minutes, and 16 seconds (365.2425 days). The currently accepted modern figure is 365 days, 5 hours, 48 minutes, 45 seconds. Adding a calendar day every four years, therefore, results in an excess of around 44 minutes for those four years, 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.
There is a popular tradition known as Bachelor's Day in some countries allowing a woman to propose marriage to a man on February 29. If the man refuses, he then is obliged to give the woman money or buy her a dress. In upper-class societies in
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