What is a leap day? All about Feb. 29, on the calendar every 4 years.

If you look at your calendar Thursday, you will see an unusual date: Feb. 29. That’s because 2024 is what’s known as a leap year, and while those in the Northern Hemisphere might not be thrilled to learn they’ll have to face an extra day of winter, there’s actually an important scientific reason leap years exist. Here’s what to know.

What is a leap year?

A leap year takes place roughly every four years, when an extra day is added to the Gregorian calendar, making the total duration of that year 366, not 365, days. The extra day is added at the end of the month of February. Feb. 29 is known as leap day.

How are leap days celebrated?

It’s not an official holiday in the United States. But in parts of the world, leap days are not treated as regular ol’ days: In Ireland, for example, women are encouraged to propose to their partners on leap days, flipping traditional gender roles. In parts of China, children give their parents gifts. In some countries, leap days are popular days for weddings.

Fun fact: Some other major events that occur every four years , like the U.S. presidential elections and the Summer Olympics, also typically take place during leap years.

Why do leap years exist?

Leap years happen because of a “mismatch between the calendar year and Earth’s orbit,” according to NASA. While we think of one year as lasting 365 days, it actually takes a little longer, approximately 365 days and 6 hours, for the Earth to orbit the sun. That extra quarter of a day has to go somewhere, so around every four years, we add 24 hours, or four times six hours, to the calendar.

As The Washington Post reported, leap days are what keep our seasons and our calendars in sync, which, in turn, makes it possible for farmers to grow crops and for humans to celebrate religious and other holidays around the same time each year. Shauna Edson, an astronomy educator at the National Air and Space Museum, previously told The Post that without them, our calendars would be ahead of the Earth’s orbit by a day every four years, amounting to 24 days each century. This means that, slowly but surely, our seasons and solstices would occur at vastly different times than we are used to. “At first, we might not notice,” Edson said. “But there eventually would be a time that we’d be celebrating the Fourth of July and it’d be snowing.”

Who came up with leap day?

According to the National Geographic, the mismatch between human calendars and the orbit of the Earth and the sun caused “chaos” for centuries, leading ancient societies in Egypt and China, and later the Roman Empire, to come up with workarounds. But nothing quite worked, until Pope Gregory XIII implemented the Gregorian calendar and its leap-year system.

Not everyone uses the Gregorian calendar, and some cultures have come up with different ways to account for the extra time it takes the Earth to orbit the sun. The Hebrew calendar, for example, has an extra month on certain years within a 19-year period, to ensure that religious holidays align with solar seasons.

Why do leap years take place (almost) every four years?

While leap years typically happen every four years, there are exceptions. (For those who want the math, that’s because it actually takes about 365.2422 days, not 365.25, for the Earth to orbit the sun, so by adding an extra day every four years, we are adding slightly more time than needed to align a complete orbit with the length of a calendar year.) According to the National Air and Space Museum, if we added a leap day every four years, we would “actually make the calendar longer by over 44 minutes. Over time, these extra 44+ minutes would also cause the seasons to drift in our calendar.”

Instead, the museum explains, “the rule is that if the year is divisible by 100 and not divisible by 400, leap year is skipped.” For example, the year 1900, which is divisible by 100 but not by 400, was skipped, but the year 2000 was not. The next time we will skip a leap year will be in 2100.

What happens if you were born on a leap day?

If you were born on a leap day, fear not, you do not enter a magical time-warp, and you still get to celebrate your birthday each year. “Leaplings,” as babies born on Feb. 29 are sometimes known, can simply choose to celebrate their birthday on March 1 on non-leap years. If that’s you, happy (slightly late) birthday!