What is leap day? Is 2024 a leap year? Everything you need to know about Feb. 29

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.

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.