February 29

  2016 (Monday)
  2012 (Wednesday)
  2008 (Friday)

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 2012, 2016, 2020, 2024, and 2028. 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.[1]

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.

A leap day is observed because the Earth's period of orbital revolution around the Sun takes approximately 6 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 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.[1][2]

Leap years

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), which had been known since the 2nd century BC when Hipparchus stated that it lasted 365 + 1/41/300 days,[3] 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 14/60 33/3600 days or 365 + 1/43/400 days or 365.2425 days.[4] The length of the tropical year in 2000 was 365.24217 mean solar days,[5] 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 common years.

Modern (Gregorian) calendar

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 leaps days fall in order Thursday, Tuesday, Sunday, Friday, Wednesday, Monday, and Saturday; then repeating with Thursday again.

Early Roman calendar

Adding a leap day (after 23 February) shifts the commemorations in the 1962 Roman Missal.

The 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 third-century writer Censorinus says:

When it was thought necessary to add (every two years) an intercalary month of 22 or 23 days, so that the civil year should correspond to the natural (solar) year, this intercalation was in preference made in February, between Terminalia [23rd] and Regifugium [24th].[6]

Julian reform

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.[7] 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.[8]

Other Languages
Afrikaans: 29 Februarie
Alemannisch: 29. Februar
አማርኛ: 29 February
Аҧсшәа: Жәабран 29
العربية: 29 فبراير
aragonés: 29 de febrero
arpetan: 29 fevriér
asturianu: 29 de febreru
Avañe'ẽ: 29 jasykõi
azərbaycanca: 29 fevral
Bân-lâm-gú: 2 goe̍h 29 ji̍t
Basa Banyumasan: 29 Februari
башҡортса: 29 февраль
беларуская: 29 лютага
беларуская (тарашкевіца)‎: 29 лютага
भोजपुरी: 29 फरवरी
Bikol Central: Pebrero 29
български: 29 февруари
bosanski: 29. februar
brezhoneg: 29 C'hwevrer
català: 29 de febrer
Чӑвашла: Нарăс, 29
čeština: 29. únor
Cymraeg: 29 Chwefror
davvisámegiella: Guovvamánu 29.
Deutsch: 29. Februar
ދިވެހިބަސް: ފެބްރުއަރީ 29
Ελληνικά: 29 Φεβρουαρίου
emiliàn e rumagnòl: 29 ed fervèr
español: 29 de febrero
Esperanto: 29-a de februaro
estremeñu: 29 hebreru
euskara: Otsailaren 29
فارسی: ۲۹ فوریه
Fiji Hindi: 29 February
føroyskt: 29. februar
français: 29 février
Gaeilge: 29 Feabhra
Gàidhlig: 29 an Gearran
贛語: 2月29號
ગુજરાતી: ફેબ્રુઆરી ૨૯
客家語/Hak-kâ-ngî: 2-ngie̍t 29-ngit
хальмг: Лу сарин 29
한국어: 2월 29일
հայերեն: Փետրվարի 29
हिन्दी: २९ फ़रवरी
hornjoserbsce: 29. februara
hrvatski: 29. veljače
Bahasa Hulontalo: 29 Pebruari
Ilokano: Pebrero 29
বিষ্ণুপ্রিয়া মণিপুরী: ফেব্রুয়ারী ২৯
Bahasa Indonesia: 29 Februari
interlingua: 29 de februario
Interlingue: 29 februar
íslenska: 29. febrúar
italiano: 29 febbraio
עברית: 29 בפברואר
Basa Jawa: 29 Fèbruari
kalaallisut: Februaari 29
Kapampangan: Pebreru 29
къарачай-малкъар: 29 февраль
ქართული: 29 თებერვალი
kaszëbsczi: 29 gromicznika
қазақша: 29 ақпан
Kiswahili: 29 Februari
Kreyòl ayisyen: 29 fevriye
Latina: 29 Februarii
latviešu: 29. februāris
Lëtzebuergesch: 29. Februar
lietuvių: Vasario 29
Limburgs: 29 fibberwarie
Livvinkarjala: 29. tuhukuudu
lumbaart: 29 02
magyar: Február 29.
मैथिली: २९ फरबरी
македонски: 29 февруари
Malagasy: 29 Febroary
മലയാളം: ഫെബ്രുവരി 29
მარგალური: 29 ფურთუთა
مصرى: 29 فبراير
Bahasa Melayu: 29 Februari
Mìng-dĕ̤ng-ngṳ̄: 2 nguŏk 29 hô̤
монгол: 2 сарын 29
မြန်မာဘာသာ: ၂၉ ဖေဖော်ဝါရီ
Nāhuatl: Tlaōnti 29
Nederlands: 29 februari
Nedersaksies: 29 febrewaori
नेपाल भाषा: फेब्रुवरी २९
日本語: 2月29日
Napulitano: 29 'e frevaro
нохчийн: 29 февраль
norsk nynorsk: 29. februar
Nouormand: 29 Févri
олык марий: 29 пургыж
oʻzbekcha/ўзбекча: 29-fevral
ਪੰਜਾਬੀ: ੨੯ ਫ਼ਰਵਰੀ
پنجابی: 29 فروری
پښتو: 29 فبروري
Перем Коми: Февраль 29’ лун
Plattdüütsch: 29. Februar
polski: 29 lutego
Ποντιακά: 29 Κούντουρονος
português: 29 de fevereiro
Ripoarisch: 29. Febrowaa
română: 29 februarie
русиньскый: 29. фебруар
русский: 29 февраля
саха тыла: Олунньу 29
Gagana Samoa: Fepuari 29
Scots: 29 Februar
Sesotho sa Leboa: Dibokwane 29
shqip: 29 shkurt
sicilianu: 29 di frivaru
Simple English: February 29
SiSwati: 29 iNdlóvana
slovenčina: 29. február
slovenščina: 29. februar
ślůnski: 29 lutygo
کوردی: ٢٩ی شوبات
српски / srpski: 29. фебруар
srpskohrvatski / српскохрватски: 29. 2.
Basa Sunda: 29 Pébruari
svenska: 29 februari
Tagalog: Pebrero 29
Taqbaylit: 29 fuṛaṛ
татарча/tatarça: 29 февраль
తెలుగు: ఫిబ్రవరి 29
тоҷикӣ: 29 феврал
Türkçe: 29 Şubat
Türkmençe: 29 fewral
українська: 29 лютого
اردو: 29 فروری
ئۇيغۇرچە / Uyghurche: 29- ھۇد
Vahcuengh: 2 nyied 29 hauh
vèneto: 29 de febraro
Tiếng Việt: 29 tháng 2
Volapük: Febul 29
West-Vlams: 29 februoari
Winaray: Pebrero 29
Yorùbá: 29 February
粵語: 2月29號
Zazaki: 29 Sıbate
Zeêuws: 29 feberwari
žemaitėška: Vasarė 29
中文: 2月29日