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.

Leap day deals 2024:Get discounts and free food from Wendy's, Chipotle, Krispy Kreme, more

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.

How often is leap year?Here's the next leap day after 2024 and when we'll (eventually) skip one

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 every four years, National Geographic reports.

In ancient Rome, their calendar varied and included a 23-day intercalary month called "Mercedonius." But it was not a standalone month. Mercedonius was added to February to account for the difference between the Roman year and solar year, according to the History Channel.

When making the Julian calendar, Caesar took inspiration from the Egyptians and decided to add an extra day to the month of February every four years. The Julian calendar officially began on Jan. 1 in 45 BCE.

This method would continue over several centuries, but not without issue. Caesar's math of 365.25 days was close, but it wasn't the exact 365.242190 days the solar year contains. To be precise, Caesar "overestimated the solar year by 11 minutes," the History Channel reports. This meant the Julian calendar would be short a day every 128 years, according to National Geographic.

By the 16th century, time had shifted again and not in a good way. Major dates had changed, including Easter. The holiday is supposed to occur on the first Sunday following the first full moon on or after the spring equinox. At the time, Easter's date had moved by about 10 days.

To fix this, Pope Gregory XIII introduced the Gregorian calendar, which kept a leap day every four years but eliminated it during centurial years not divisible by 400, according to the History Channel. This is why 1700, 1800 and 1900 were not leap years, but 2000 was.

Despite its accuracy, the Gregorian calendar is not flawless. Instead of being off by one day every 128 years like the Julian calendar, the Gregorian calendar falls short once every 3,030 years, the History Channel reports.

Upcoming leap days

This year, 2024, is a leap year. Leap day will fall on Thursday, Feb. 29.

The next leap years will be in 2028, 2032 and 2036.

Leap day deals: Where to score on food, products and more Feb. 29

Food chains like Wendy's, Chipotle and Krispy Kreme are offering deals and free food in honor of leap day. Here's where to score on discounts.

What happens if you are born on leap day?

Feb. 29 is the rarest birthday someone could have. Still, at least 5 million people celebrate their birthday on leap day, according to the History Channel. Your odds of being born on Feb. 29 are one-in-1,461.

Many "Leaplings" (or those born on leap day) will celebrate their birthdays on Feb. 28 or March 1 during a typical 365-day year, even though documents will reflect it is on Feb. 29.

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