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 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. 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. It is the last day of February in leap years, with the exception of 1712 in Sweden. It is also the last day of meteorological winter in the Northern Hemisphereand the last day of meteorological summer in the Southern Hemisphere in leap years.
In the Gregorian calendar, the standard civil calendar used in most of the world, February 29 is added in each year that is an integer multiple of four unless it is evenly divisible by 100 but not by 400. For example, 1900 was not a leap year but 2000 was. The Julian calendar—since 1923 a liturgical calendar—has a February 29 every fourth year without exception. Consequently, February 29 in the Julian calendar, since 1900, falls 13 days later than February 29 in the Gregorian, until the year 2100.
A leap day is observed because the Earth’s period of orbital revolution around the Sun takes approximately six hours longer than 365 whole days. A leap day compensates for this lag, realigning the calendar with the Earth’s position in the Solar System; otherwise, seasons would occur later 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 three 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 aforementioned century rule to keep the equinoxes more or less fixed and the date of Easter consistently close to the vernal equinox. Leap days can present a particular problem in computing known as the Leap year bug, when February 29 is not handled correctly in logic that accepts or manipulates dates,
Although most modern calendar years have 365 days, a complete revolution around the Sun (one solar year) takes approximately 365 days, 5 hours, 48 minutes, and 46 seconds (or, for simplicity’s sake, approximately 365 days and 6 hours, or 365.25 days). An extra 23 hours, 15 minutes, and 4 seconds thus accumulates every four years (again, for simplicity’s sake, approximately an extra 24 hours, or 1 day, 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.
Solar years are actually slightly shorter than 365 days and 6 hours (365.25 days), which had been known since the 2nd century BC when Hipparchus stated that it lasted 365 + Days but this was ignored by Julius Caesar and his astronomical adviser Sosigenes. The Gregorian calendar corrected this by adopting the length of the tropical year stated in three medieval sources, the Alfonsine tables, De Revolutionibus, and the Prutenic Tables, truncated to two sexagesimal places, 365 days or 365.2425 days. The length of the tropical year in 2000 was 365.24217 mean solar days, Adding a calendar day every four years, therefore, results in an excess of around 44 minutes every 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 is also 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 (146,097 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. Excepting when a century mark that is not a multiple of 400 intervenes, consecutive leap days fall in order Sunday, Friday, Wednesday, Monday, Saturday, Thursday, and Tuesday; then repeating with Sunday again.
The Early Roman calendar of the Roman king Numa Pompilius had only 355 days (even though it was not a lunar calendar) which meant that it would quickly become unsynchronized with the solar year. An earlier Roman solution to this problem was to lengthen the calendar periodically by adding extra days to February, the last month of the year. February consisted of two parts, each with an odd number of days. The first part ended with the Terminalia on the 23rd, which was considered the end of the religious year, and the five remaining days formed the second part. To keep the calendar year roughly aligned with the solar year, a leap month, called Mensis Intercalaris (“intercalary month”), was added from time to time between these two parts of February. The (usual) second part of February was incorporated in the intercalary month as its last five days, with no change either in their dates or the festivals observed on them. This followed naturally, because the days after the Ides (13th) of February (in an ordinary year) or the Ides of Intercalaris (in an intercalary year) both counted down to the Kalends of March (i.e. they were known as “the nth day before the Kalends of March”). The Nones (5th) and Ides of Intercalaris occupied their normal positions.
The set leap day was introduced in Rome as a part of the Julian reform in the 1st century BC. As before, the intercalation was made after February 23. The day following the Terminalia (February 23) was doubled, forming the “bis sextum”—literally ‘twice sixth’, since February 24 was ‘the sixth day before the Kalends of March’ using Roman inclusive counting (March 1 was the Kalends of March and was also the first day of the calendar year). Inclusive counting initially caused the Roman priests to add the extra day every three years instead of four; Augustus was compelled to omit leap years for a few decades to return the calendar to its proper position. Although there were exceptions, the first day of the bis sextum (February 24) was usually regarded as the intercalated or “bissextile” day since the 3rd century AD.[10] 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,[citation needed] although this has only been formally enacted in Sweden and Finland. In Britain, the extra day added to leap years remains notionally the 24th, although the 29th remains more visible on the calendar.
A person born on February 29 may be called a “leapling”, a “leaper”, or a “leap-year baby”. Some leaplings celebrate their birthday in non-leap years on either February 28 or March 1, while others only observe birthdays on the authentic intercalary date, February 29. 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 Europe, if the man refuses marriage, he then must purchase 12 pairs of gloves for the woman, suggesting that the gloves are to hide the woman’s embarrassment of not having an engagement ring. In Ireland, the tradition is supposed to originate from a deal that Saint Bridget struck with Saint Patrick. In the town of Aurora, Illinois, single women are deputized and may arrest single men, subject to a four-dollar fine, every February 29. In Greece, it is considered unlucky to marry on a leap day