A leap year is a year with 366 days instead of the usual 365. The extra day, February 29, gets added to keep our calendar aligned with Earth's actual journey around the sun.

Here's why we need it: Earth takes about 365 and a quarter days to orbit the sun completely. If we ignored that quarter day, our calendar would slowly drift out of sync with the seasons. After 100 years without leap years, summer would arrive several weeks earlier on the calendar than it should.

The solution? Add an extra day every four years to account for those accumulated quarter days. Most years divisible by four are leap years: 2024, 2028, 2032. There's one quirk: century years (like 1900 or 2000) are only leap years if they're divisible by 400. So 2000 was a leap year, but 1900 wasn't.

People born on February 29 are called leaplings or leapers. They technically have birthdays only once every four years, though they usually celebrate on February 28 or March 1 during non-leap years. Being a leapling means you could turn 12 years old but have experienced only three actual birthdays, which makes for interesting conversations about age.

The term leap year comes from the fact that dates “leap” forward in these years. In a regular year, if your birthday falls on a Tuesday, it will fall on a Wednesday the next year. But in a leap year, it leaps over Wednesday and lands on Thursday.