February 29, also known as Leap Day or Leap Year Day, is a date added to most years that are divisible by 4, like 2020. However, if a year is divisible by 100, but not by 400, it does not contain a Leap Day. Thus, 1700, 1800, and 1900, did not contain a Leap Day but 1600 and 2000 did.
A Leap Day is observed because the Earth’s 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 and eventually spring would be in July.
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.
Leap days can present a particular problem in computing known as the Lead Year Bug, when February 29th is not handled correctly in logic that accepts or manipulates dates. A person born on February 29 may be called a “leapling”, a “leaper”, or a “leap-year baby”.
A popular myth is that women may propose to men on February 29.