Once every four years, our 365-day rotation around the sun becomes 366.

2024 is a leap year, meaning we will add one day to the end of February and therefore extend the year by one. Since leap year happens every four years, our last leap days were in 2020 and 2016, and the next leap year will happen in 2028.

Here's what to know about leap year, when to expect it and why it's something that falls on our calendars once every four years.

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When is leap day?

Leap day is on Feb. 29, 2024.

February, our shortest month of the year, typically has 28 days on the calendar. But in a leap year, we add one more day to February, making it 29 days long.

The last leap day was in 2020, and the next one will be in 2028.

What is leap day?

While nothing particularly notable happens on leap day (beyond making February one day longer), the reason why we do it comes down to science.

Our normal calendar years are typically 365 days long, or the number of days it takes Earth to orbit the sun. But according to the National Air and Space Museum, 365 days is a rounded number. It actually takes 365.242190 days for the Earth to orbit completely, or 365 days, 5 hours, 48 minutes and 56 seconds.

By adding an extra day every four years, this allows our calendar to stay on track with the Earth's actual orbit and not have our seasons drift, as our equinoxes and summer and winter solstices would no longer align with the seasons.

How often do leap days occur? Not all are every four years

Given that leap years are supposed to occur every four years, that would make sense. But it's not that simple, the National Air and Space Museum says. When we add a leap day every four years, we make our calendar longer by 44 minutes, and over time, that also causes seasons to drift.

To combat this, the rule is that if the year is divisible by 100 but not 400, we skip that leap year. We skipped leap years in 1700, 1800 and 1900, but we did not skip it in 2000.

The next leap year we'll skip will be in 2100.

Why is leap day in February?

Choosing February for the leap year and the addition of an extra day dates back to the reforms made to the Roman calendar by Julius Caesar, who was inspired by the Egyptian solar calendar, according to History.com. The Roman calendar, at that time, was based on a lunar system and had a year of 355 days, which was shorter than the solar year. This discrepancy caused the calendar to drift out of sync with the seasons over time.

To address this issue, Julius Caesar introduced the Julian calendar, a solar calendar, which included a leap year system. When the Julian calendar was later refined into the Gregorian calendar in 1582, the tradition of adding a leap day to February persisted.

Contributing: Saman Shafiq, USA TODAY.