Adding an extra day every four years keeps our calendar aligned correctly with the astronomical seasons, since a year according to the Gregorian calendar (365 days) and a year according to Earth’s orbit around the Sun (approximately 365.25 days) are not the same length of time. Without this extra day, our calendar and the seasons would gradually get out of sync. (Keep reading for a longer explanation.)
Because of this extra day, a leap year has 366 days instead of 365. Additionally, a leap year does not end and begins on the same day of the week as a non–leap year does.
How Do You Know If It’s a Leap Year?
Generally, a leap year happens every four years, which, thankfully, is a relatively simple pattern to remember. However, there is a little more to it than that.
Here are the rules of leap years:
A year may be a leap year if it is evenly divisible by 4.
Years divisible by 100 (century years such as 1900 or 2000) cannot be leap years unless they are also divisible by 400. (For this reason, the years 1700, 1800, and 1900 were not leap years, but the years 1600 and 2000 were.)
If a year satisfies both the rules above, it is a leap year.
When Is the Next Leap Year? When is the Next Leap Day?
Thursday, February 29
Tuesday, February 29
Sunday, February 29
Friday, February 29
Why Do We Need Leap Years?
The short explanation for why we need leap years is that our calendar needs to stay aligned with the astronomical seasons.
One orbit of Earth around the Sun takes approximately 365.25 days—a little more than our Gregorian calendar’s nice, round number of 365. Because the calendar does not account for the extra quarter of a day that the Earth requires to complete its orbit around the Sun, it doesn’t completely align with the solar year.
Because of this .25 difference, our calendar gradually gets out of sync with the seasons. Adding an extra day, aka a “leap day,” to the calendar every four years brings the calendar in line and therefore realigns it with the seasons.
Without leap days, the calendar would be off by 5 hours, 48 minutes, and 45 seconds more each year.
After 100 years, the seasons would be off by 25 days! Eventually, the months we call February and March would feel like summer in the Northern Hemisphere.
The extra leap day adjusts this drift, but it’s not a perfect match: Adding a leap day every four years overcompensates by a few extra seconds each leap year, adding up to about three extra days every 10,000 years.
What Is a Leap Day? And a Leapling?
A “leap day” is the extra day in the leap year: February 29.
A “leapling” is a person born on a leap day. Any leap-day babies out there? We’d love to hear from you in the comments below!
Leap Year Facts and Folklore
Ages ago, Leap Day was known as “Ladies Day” or “Ladies’ Privilege,” as it was the one day when women were free to propose to men. Today, Sadie Hawkins Day sometimes applies to Feb 29 (leap day), based on this older tradition.
According to folklore, the weather always changes on Friday in a leap year.
“Leap year was ne’er a good sheep year” (old proverb)
Are Leap Years Bad Luck?
Many feel that to be born on Leap Day, thereby becoming a “leapling,” is a sign of good luck.
In some cultures, getting married during a leap year is considered bad luck.
We don’t know of any evidence supporting that marriage theory, but we do know that during leap years:
Rome burned (64),
and the Titanic sank (1912).
Also, in leap years:
The Pilgrims landed at Plymouth, Massachusetts (1620),