What is a leap year? Everything to know about February's bonus day

With holidays like Valentine's Day, Presidents Day, Groundhog Day and Super Bowl Sunday (yes, an unofficial one, but still worth celebrating), February is arguably one of the most festive months of the year.

With just 28 days, it's also the shortest.

For some, the second month of the calendar year comes as a relief, because fewer days can equate to lower utility bills. For others, February can also mean a smaller paycheck since there aren't as many hours to work as, say, October or May, which both have 31 days.

But once every four years, February gets a bonus day and when it happens, it's known as a leap year.

What is a leap year? Well, it has to do with how many days there are in an average year (which is 365) and the earth's rotation around the sun. In fact, it all boils down to simple math.

To help you better understand Feb. 29, or leap day, we've got all the details on the meaning of leap year, how often it happens, and why it's on the calendar in 2024.

So, read on to learn about this year's extra day and don't forget to celebrate or make some fun plans to commemorate this once-every-four-year event.

What is a leap year and why was it created?

The standard Gregorian calendar has 365 days, which lines up with the earth's orbit around the sun, otherwise known as a solar year.

However, according to NASA, it actually takes the earth 365.242 days to make its way around around the sun. Did you catch the .242 part? Technically, there's an extra quarter of day (or six hours) each year unaccounted for on the calendar and for the sake of convenience, it's simply rounded down to 365.

But over time, those extra six hours add up and by year four, they've accumulated to equal 24 hours or an entire day. That extra day has to be made up somewhere or eventually seasons would shift enough that, as NASA puts it, we'd be celebrating July 4 in the middle of winter.

To correct the problem, once every four years an extra day is added to the month of February and, you guessed it, it's known as leap day.

Because of that, in 2024 there will be 366 days instead of the typical 365.

What is a leap century year?

Perhaps you already know the reason behind February's extra day, but here's a bit of trivia you may not.

According to the National Institute of Standards and Technology (NIST), there's an exception to the leap year rule and it involves a slightly tricky calculation.

Remember the 365.25? In reality, the average solar year is just a smidge less than that. Over time, that infinitesimal amount adds up, resulting in roughly three days being added to the calendar every 400 years.

What that means is that one of out every four centuries is considered a leap century year.

Here's the confusing part: According to the NIST, century leap years are only leap years if they can be evenly divided by 400. So, for example, 1700, 1800 and 1900 weren't leap years. And 2100? It's not a leap century year either since it isn't divisible by 400.

However, because they can be evenly divided by 400, the years 1600 and 2020, were both leap century years.

If you want to mark your calendar, the next leap century year will occur in 2400.

When is the next leap year?

The last time the calendar read February 29 was in 2020.

If you've been keeping up, it's pretty simple to figure out the next round of leap years.

After 2024, the next time February will have 29 days will be in 2028 and then again in 2032.

This article was originally published on TODAY.com