Although most years of the modern calendar have 365 days, a complete revolution around the sun takes approximately 365 days and 6 hours. Every four years, during which an extra 24 hours have accumulated, one extra day is added to keep the count coordinated with the sun's apparent position.
It is, however, slightly inaccurate to calculate an additional 6 hours each year. A better approximation, derived from the Alfonsine tables, is that the Earth takes a complete revolution around the sun in 365 days, 5 hours, 49 minutes, and 16 seconds. To compensate for the difference, an end-of-century year is not a leap year unless it is also exactly divisible by 400. This means that the years 1600 and 2000 were leap years, as will be 2400 and 2800, but the years 1700, 1800 and 1900 were not, nor will be 2100, 2200 and 2300.
The Gregorian calendar repeats itself every 400 years, which is exactly 20,871 weeks including 97 leap days. Over this period, February 29 falls 13 times on a Sunday, Tuesday, or Thursday; 14 times on a Friday or Saturday; and 15 times on a Monday or Wednesday.
The concepts of the leap year and leap day are distinct from the leap second, which results from changes in the Earth's rotational speed. Adding a leap day (after 23 February) shifts the commemorations in 1962 Roman Missal.
The leap day was introduced as part of the Julian reform. The day following the Terminalia (February 23) was doubled, forming the "bis sextum"—literally 'double sixth', since February 24 was 'the sixth day before the Kalends of March' using Roman inclusive counting (March 1 was the 'first day'). Although exceptions exist, the first day of the bis sextum (February 24) was usually regarded as the intercalated or "bissextile" day since the third century. 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.
In the United Kingdom, a person born on February 29 legally attains the age of 18 on March 1 in the relevant year. In the European Union, February 29 officially became the leap day only in 2000.
In cases of New Zealand citizens, the NZ Parliament has decreed that if a date of birth was February 29, in non-leap years the legal birth date date shall be the preceding day, the 28th. This is affirmed in §2(2) of the Land Transport Act 1999.
In France, there is a humorous periodical called La Bougie du Sapeur (The Sapper's Candle) published every February 29 since 1980. The name is a reference to the sapper Camember, a comic strip character born February 29, 1844 who was created by Georges Colomb in the 1890s.
* 1504 – Christopher Columbus uses his knowledge of a lunar eclipse that night to convince Native Americans to provide him with supplies.
* 1704 – Queen Anne's War: French forces and Native Americans stage a raid on Deerfield, Massachusetts, killing 56 villagers and taking more than 100 captive.
* 1712 – February 29 is followed by February 30 in Sweden, in a move to abolish the Swedish calendar for a return to the Old style.
* 1720 – Queen Ulrika Eleonora of Sweden abdicates in favour of her husband, who becomes King Frederick I.
* 1796 – The Jay Treaty between the United States and Great Britain comes into force, facilitating ten years of peaceful trade between the two nations.
* 1864 – American Civil War: Kilpatrick-Dahlgren Raid fails – plans to free 15,000 Union soldiers being held near Richmond, Virginia are thwarted.
* 1892 – St. Petersburg, Florida is incorporated.
* 1916 – Child labor: In South Carolina, the minimum working age for factory, mill, and mine workers is raised from twelve to fourteen years old.
* 1932 – TIME magazine features eccentric American politician William "Alfalfa" Murray on its cover after Murray stated his intention to run for President of the United States.
* 1936 – Baby Snooks, played by Fanny Brice, debuts on the radio program The Ziegfeld Follies of the Air.
* 1940 – For her role as Mammy in Gone with the Wind, Hattie McDaniel becomes the first African American to win an Academy Award.
* 1940 – Finland initiates Winter War peace negotiations
* 1940 – In a ceremony held in Berkeley, California, because of the war, physicist Ernest Lawrence receives the 1939 Nobel Prize in Physics from Sweden's Consul General in San Francisco.
* 1944 – World War II: The Admiralty Islands are invaded in Operation Brewer led by American General Douglas MacArthur.
* 1952 – The island of Heligoland is restored to German authority.
* 1956 – U.S. President Dwight D. Eisenhower announces to the nation that he is running for a second term.
* 1960 – An earthquake in Morocco kills over 3,000 people and nearly destroys Agadir in the southern part of the country.
* 1960 – Family Circus makes its debut.
* 1964 – In Sydney, Australian swimmer Dawn Fraser sets a new world record in the 100-meter freestyle swimming competition (58.9 seconds).
* 1972 – Vietnam War: Vietnamization – South Korea withdraws 11,000 of its 48,000 troops from Vietnam.
* 1972 – Hank Aaron becomes the first player in the history of Major League Baseball to sign a $200,000 contract.
* 1980 – Gordie Howe of the then Hartford Whalers makes NHL history as he scores his 800th goal.
* 1984 – Canadian Prime Minister Pierre Trudeau announces he will retire as soon as the Liberals can elect another leader.
* 1988 – South African archbishop Desmond Tutu is arrested along with 100 clergymen during a five-day anti-apartheid demonstration in Cape Town
* 1988 – Svend Robinson becomes the first member of the Canadian House of Commons to come out as gay.
* 1996 – Faucett Flight 251 crashes in the Andes, killing 123 people.
* 2004 – Jean-Bertrand Aristide is removed as President of Haiti following a coup.