Tomorrow is February 29th, which we only get about once every four years.

Here are five things you should know about tomorrow's Leap Day:

1. We need Leap Days because it takes the Earth slightly longer than 365 days to circle the sun. It actually takes 365 days, five hours, 48 minutes and 45 seconds.

So without Leap Years, we'd be off by about six hours every year. That doesn't sound like much, but over 100 years, it adds up to nearly 25 whole days.

2. It's not as simple as adding a day every four years. For a particular year to be a Leap Year, three criteria must be met: One, the year must be evenly divisible by 4. Two, if the year can ALSO be evenly divided by 100, it's not a Leap Year. And three, if the year is evenly divisible by 400 then it is a Leap Year.

That's why the year 2000 was a Leap Year, since it's divisible by 400, but 2100 won't be.

3. In addition to Leap Days and Leap Years, there's also a Leap Second. The last one was added on December 31st, 2016, and the next one might be added on December 31st of this year.

4. Less than 0.07% of the world's population is estimated to have been born on February 29th, which means only about 205,000 people in the U.S. have a birthday tomorrow. (Or about 4.8 million people in the whole world.)

5. People born during a Leap Year on February 29th are known as "leaplings," "leapers," or "leapsters" and some famous leapers include Tony Robbins, Antonio Sabato Jr., Dennis Farina, and Ja Rule.