Tomorrow (Thursday, 2/29/24) is Leap Day! (Almost) every four years, February becomes one day longer, making the month 29 days long. 

Why do we have leap years? Well, unlike what our calendar says, the Earth actually takes slightly longer than 365 days to rotate fully around the Sun. So if we ignored the fact that those extra hours actually exist consistently every single year, our seasons would eventually become off. For example, if summer traditionally starts in June in the northern hemisphere, in less than 1000 years, summer could instead start in December. So to avoid all of this, we add a year to the calendar almost every four years. 

To understand why we don’t have leap years every four years, we first must understand a little about the Roman, Julian, and Gregorian Calendars.

The Roman Calendar was created by Romulus, the founder of Rome, in the 8th century B.C. It originally had 10 months in total and counted all of winter as its own “month”. However, in the year 46 B.C., Julius Caesar established the Julian Calendar which included January and February. For common years, it had 365 days and it had a leap year consistently every four years, making it have a 366th day at the end of February.

In 1582, Pope Gregory XIII created the Gregorian Calendar. Which fixed one problem: the fact that if we have a leap year exactly every four years, our calendar years would still be longer than the actual rotation of the Earth around the Sun by 44 minutes. So, the Pope created a rule that “if the year is divisible by 100 and not divisible by 400, leap year is skipped,” (via Smithsonian). So for example, we had a leap year in the years 1800 and 1900, but not 2000 because of this rule.