What is a leap year and why do we have them? What to know for 2024

While there are plenty of things to note in the calendar for 2024, one of the biggest is that this year is a leap year.

But what exactly does that mean, how often does it happen and why?

Here's what you should know now that 2024 has officially begun.

What is a leap year?

A leap year means there's an extra day in the calendar.

"It takes approximately 365.25 days for Earth to orbit the Sun — a solar year. We usually round the days in a calendar year to 365. To make up for the missing partial day, we add one day to our calendar approximately every four years. That is a leap year," according to NASA.

What is a leap day?

During a leap year, the month of February has an extra day added to it. So this year, there will be 29 days in February, rather than 28. That 29th day is also referred to as a "leap day."

When are leap years and why don't they happen every four years exactly?

Leap years happen almost every four years. That means 2024 and 2028 will both be leap years.

But still, there are some exceptions to the once-every-four-years rules. That's because "it takes Earth approximately 365.242189 days, or 365 days, 5 hours, 48 minutes, and 45 seconds, to circle once around the Sun," according to Time and Date.

"If the tropical year was precisely six hours longer than a calendar year with 365 days, we could use the Julian calendar, which adds a leap day every 4 years without exception. The deviation would grow to exactly 24 hours over 4 years, and Earth would need exactly one day to catch up to the position in its orbit where it was four years prior," the publication states. "However, the deviation between the common year and the tropical year is a little less than six hours. The Gregorian calendar addresses this by employing a slightly more complicated set of rules to determine which years are leap years. It's still not perfect, but the resulting deviation is very small."

How can you calculate a leap year?

There is a set of rules for determining whether a year is a leap year. And, according to the Farmer's Almanac, those are:

1. A year may be a leap year if it is evenly divisible by 4.
2. Years divisible by 100 (century years such as 1900 or 2000) cannot be leap years unless they are also divisible by 400. (For this reason, the years 1700, 1800, and 1900 were not leap years, but the years 1600 and 2000 were.)

What if you are born during a leap year?

According to the Almanac, people born on a leap day during a leap year are known as "leaplings." Some cultures see the birthday as a sign of good luck.