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2024 is a leap year, meaning we will have a 29th day tacked on to the end of this month. Occurring only once every four years, leap years describe the practice of adding an extra day to the year to align our calendar properly with Earth’s orbit.

Occasionally, we skip a year to keep the science in check, though we haven’t done so since the 20th century.

This year, leap day falls on a Thursday. Wondering when the next one will be? Here’s what to know about leap years past and future.

What is a leap year?

Leap years happen when we add one day to the end of February in order to align our calendar with the Earth’s orbit. Once every four years, we tack a 29th day onto the end of February, which is usually 28 days long, making a leap year 366 days instead of 365.

We generally refer to 365 as the number of days it takes for the Earth to orbit the sun, but it actually takes 365.242190 days or 365 days, 5 hours, 48 minutes and 56 seconds, to be exact.

When is leap day 2024?

Leap day occurs on Thursday, Feb. 29.

Before 2024, the last leap year was in 2020.

When is the next leap day?

After 2024, the next leap year will happen in 2028 and will fall on Tuesday, Feb. 29.

How often are leap years?

Leap years occur every four years, with an exception. Occasionally, we skip what is supposed to be a leap year for the same reason we have them in the first place. Adding a leap day once every four years eventually results in our calendar becoming 44 minutes too long, which can also knock our seasons and calendar off whack.

As a result, we do skip leap years, though we do so at intervals much larger than four years. 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.