2024 is a leap year, meaning Thursday, Feb. 29 is a once-in-every-four-year event. Since leap years typically happen every four years (although there are some exceptions), our last leap days were in 2020 and 2016, and the next leap year will happen in 2028. And since this is a day that doesn’t come around often, people are celebrating in different ways, with businesses offering special deals in commemoration and others finally celebrating their Feb. 29 birthday.

Here’s everything you need to know about leap day, including what is it, why it comes every four years and when it was created.

What is leap day?

Leap day is an extra day that gets added to the calendar. During a leap year, which occurs every four years, leap day falls on Feb. 29, giving the shortest month of the year one added day.

Why is leap day every four years?

The reason there are leap days, and years, is because of the Earth’s orbit. The amount of days it takes for the Earth to complete a full revolution around the Sun is not a whole number. The 365 days we experience is actually 365.242190 days, according to the National Air and Space Museum. Getting rid of those 0.242190 days adds up. That fraction allows seasons to correctly line up each year. If leap day was left off the calendar, the months during which we normally experience each season would eventually shift. This would impact other aspects of life, such as the growing and harvesting of crops. When added, four 0.242190 days roughly equal one full day, which is why Feb. 29 is added to the calendar of most years that are divisible by four, including 2024.

When do we skip leap day?

To make up for decimals of time, we’ll sometimes skip leap years, but it’s rare. Prepare for a little bit of math: years divisible by 100 but not 400 are skipped, meaning we skipped leap years in 1700, 1800 and 1900 but not 2000. The next leap year we’ll skip is quite a ways away, in 2100.

Who created leap day?

The concept of adding leap days is not new and has been around for millennia, Britannica reports. Some calendars – such as the Hebrew, Chinese and Buddhist calendars – contained leap months, also known as ‘intercalary or interstitial months,’ according to the History Channel. While Julius Caesar is often credited for originating leap days, he got the idea from the Egyptians. By the third-century BCE, Egyptians followed a solar calendar that spanned 365 days with a leap year ever