This website uses cookies

We use cookies to give you the best experience possible. By continuing we’ll assume you’re on board with our cookie policy.

Leap year rules


For centuries, humans struggled to sync civil, religious, and agricultural calendars with the solar year. Adding a 'leap year' solved the problem

Nearly every four years, we add an extra day to the calendar in the form of February 29, also known as Leap Day. Put simply, these additional 24 hours are built into the calendar to ensure that it stays in line with the Earth’s movement around the Sun. While the modern calendar contains 365 days, the actual time it takes for Earth to orbit its star is slightly longer, roughly 365.2421 days. The difference might seem negligible, but over decades and centuries that missing quarter of a day per year can add up. To ensure consistency with the true astronomical year, it is necessary to periodically add in an extra day to make up the lost time and get the calendar back in synch with the heavens. 

Julius Caesar was behind the origin of leap year. While regarded as one of the world’s most powerful and influential historical figures, Julius Caesar wasn’t an expert on math or the stars above. So when it became necessary to recalibrate his empire’s inadequate calendar—an undertaking that would eventually give humanity the Leap Year—he called upon a cadre of Greek experts. He tasked Sosigenes of Alexandria, a well‑regarded Greek astronomer whom Caesar met while putting down a rebellion in Egypt. The Julian calendar—named in Caesar’s honor—was introduced in 46 BCE and with it, the Leap Year in the standard once-every-four-years format. It stood as the model of the western world for hundreds of years.

Until another leader, Pope Gregory XIII, saw the same need for further recalibration. He too relied on experts to get the job done. The Gregorian calendar was introduced in the 16th century, improving on its Roman predecessor sufficiently enough to become ubiquitous.

Many Leap Day customs have revolved around romance and marriage. Tradition holds that in 5th-century Ireland, St. Bridget lamented to St. Patrick that women were not allowed to propose marriage to men. So legend has it that St. Patrick designated the only day that does not occur annually, February 29, as a day on which women would be allowed to propose to men. In some places, Leap Day thus became known as Bachelor's Day. In Greek tradition, however, it is considered bad luck to marry on Leap Day, and statistics suggest that Greek couples continue to take this superstition seriously.