Take a Leap this Leap Day!

Sunrise in Dülmen, Germany by Dietmar Rabich

Today is leap day, an extra day that occurs every four years (with some exceptions) to account for a discrepancy between the calendar year and what is called the tropical year, based on the year’s seasons. The tropical year is roughly 365.2422 days long. Leap day accounts for this extra quarter of a day each year by adding February 29th on leap years. Without leap day, the numeric drift would throw the calendar out of sync between the equinoxes and the seasons. Many different calendar systems have arisen to account for this specific problem.

Today, much of the world uses the Gregorian calendar instituted by its namesake Pope Gregory XIII in 1582. This system was a refinement of Julius Caesar’s Julian calendar that incorrectly estimated the tropical year length to be 365.25. The Gregorian calendar is a more precise 365.2425, closer to the real length of 365.2422 but rounded so as to allow for a uniform occurrence of the leap day. In the Gregorian system, there still is numeric drift, but it will be around 3,300 years before the calendar date and the tropical date is out of sync.

The opening of Gregory's papal decree 

Gregory’s reforms were vital for calculating Easter’s date (and thus a number of other related Christian holidays), which was of obvious importance to the Catholic Church. Beyond that, leap day has taken on its own special folk traditions. In Britain and Ireland, women traditionally propose marriage on leap day, based on a legendary arrangement in Irish hagiography between Saint Patrick and Saint Bridget. In Anthony, Texas, the town holds a leap day festival for those born on the date, known as leaplings.

There are some exceptions to the leap day rule. Leap days fall on every year divisible by four (2024, 2020, 2016, etc.) except the beginnings of centuries that can’t be divided by 400 (1700, 1800, 1900, etc.) Besides these rare exceptions, every four years we get one extra day to do whatever we like. Don’t forget to take your own leap on this leap day!