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While a typical year is typically composed of 365 days, every four years the calendar adds one extra day to the end of February to catch up with the solar cycle.
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That extra day, February 29, is known as "Leap Day."
Though it's the rarest day on the calendar, a number of significant events have occurred on Leap Day since since its inception in 45 B.C.
Here are the most important historical events February 29 has brought us:
45 B.C.: The first Leap Day is recognized by proclamation of Julius Caesar.
In 45 B.C., Julius Caesar was attempting to develop a 365-day calendar when he confronted a slight dilemma: The earth's orbit around the sun takes about 365 days and six hours, which would make the calendar year slightly shorter than the solar year. Not taking this into account would throw off the passage of time and changing seasons.
To remedy this, Caesar recruited an Alexandrian astronomer named Sosigenes, and together they decided to add an extra day to the calendar every four years. Though the idea seemed simple at first, after a few centuries, it was clear something was off.
Their calculations were slightly wrong. The solar year was actually a bit shorter than they had thought. So rather than catching up, the calendar was speeding ahead.
By the end of the 16th century, the calendar year had sped as much as 10 days ahead.
The introduction of the Gregorian calendar in 1582 marked the Western calendar as we now know it. The current calculation of a solar year is 365.2425 days, and though our leap year situation is still not perfect, it will be off by about one day only every 3,030 years.
1692: The first warrants were issued for the arrests of three women in the Salem Witch Trials.
On Leap Day in 1692, three women — Sarah Good, Sarah Osborne, and Tituba — were accused of witchcraft, marking the beginning of the Salem Witch Trials.
The fate of the women was bleak. Good was hanged for refusing to confess, Osborne died one year later in prison, and Tibuta, a slave, admitted to her alleged crimes and was eventually released from prison.
1736: Ann Lee, the founder of the Shaker Movement in America, is born in Manchester, UK.
Ann Lee joined the Shaker movement, a pacifist sect of Christianity, in 1758. After having a "divine realization" 12 years later, she became the leader of the religious movement in Europe.
In 1774 she received another divine message telling her to establish a Shaker church in America. Two years later, she had gathered a following in Albany, New York.
On Leap Day in 1908, Heike Kamerlingh Onnes made an announcement proclaiming that "solid helium" was observed in his laboratory. But the Dutch scientist was slightly mistaken.
Onnes, known for his groundbreaking work in liquifying helium, had been experimenting with temperature and tried to condense the gas into glass tubes. During this process, the helium appeared to solidify.
Aghast by this discovery, Onnes was quick to announce his findings the very next day. But his observations were off, and weeks later it was realized that this phenomenon occurred only because of the presence of hydrogen.
1936: The Soviet government renames the First Leningrad Medical Institute "The Pavlov Institute" two days after Ivan Pavlov's death and preserved his brain.
Ivan Pavlov became known worldwide for his psychological study of "conditioning," or training someone to behave through a learned reflex.
The researcher is held in high regard. In 1904, he received a Nobel Prize in physiology/medicine for his research on digestion.
1940: Actress Hattie McDaniel became the first African American to win an Academy Award.
Hattie McDaniel won Best Supporting Actress for her role as Mammy in "Gone with the Wind."
1964: The first royal baby born on Leap Day.
In 1964, Princess Alexandra of Kent gave birth to a son, James Ogilvy, on Leap Day. The baby is believed to be the first royal baby in history born on February 29.
1972: Baseball player Hank Aaron becomes the highest-paid player in the league.
In 1972, Hank Aaron signed a three-year contract with the Atlanta Braves for about $200,000 a year, making him the highest-paid player in the league.
1980: Buddy Holly's glasses were found, 20 years after he died in a plane crash.
Buddy Holly's famous black glasses were rediscovered by a police officer on Leap Day over two decades after the singer died in a plane crash near Clear Lake, Iowa, in 1959.
The thick-rimmed glasses had been buried in the wreckage and turned into the County Sheriff's office a year after the crash. At that time, they were put into an envelope and completely forgotten about, until February 29, 1980, when County Sheriff Jerry Allen came across the frames and returned them safely to Holly's wife.
1996: The Siege of Sarajevo is lifted.
After nearly four years of continuous attacks by Serbs, the siege of the Bosnian capital Sarajevo was finally declared over on Leap Day in 1996. This event marked the longest siege in the history of modern warfare.
2012: The Monkees singer Davy Jones dies at 66 on Leap Day.
Davy Jones was born in Manchester, England, in 1945 and gained widespread recognition as the lead singer of the pop-rock group, "The Monkees."
Throughout his career, Jones was nominated for four Grammy awards, and even stared in acting roles for series such as "The Brady Bunch."
Jones died on Leap Day in 2012 from a sudden heart attack.
