About every four years, February gains an extra day. We do this so our calendars don't get out of whack, but Feb. 29 has also prompted some interesting traditions. Here are some surprising facts about the bonus day that comes only every so often.

1. It's all about the sun

It takes the Earth about 365.242189 days — or 365 days, 5 hours, 48 minutes and 45 seconds — to circle once around the sun, says Time and Date. However, the Gregorian calendar we rely on has only 365 days, so if we didn't add an extra day to our shortest month about every four years, we would lose almost six hours every year. After a century, our calendar would be off by about 24 days.

2. Caesar and the pope

The assassination of Julius Caesar, painted by William Holmes Sullivan, c. 1888 Julius Caesar's assassination had nothing to do with his leap year math. (Photo: William Holmes Sullivan (1836-1908) [Public domain]/Wikimedia Commons)

Julius Caesar introduced the first leap year around 46 B.C., but his Julian calendar had only one rule: Any year evenly divisible by four would be a leap year. That created too many leap years, but the math wasn't tweaked until Pope Gregory XIII introduced his Gregorian calendar more than 1,500 years later.

3. Technically, it's not every 4 years

Caesar's concept wasn't bad, but his math was a little off; the extra day every four years was too much of a correction. As a result, there's a leap year every year that is divisible by four, but to qualify, century years (those that end in 00) must also be divisible by 400. So, the year 2000 was a leap year, but the years 1700, 1800 and 1900 were not.

4. Popping the question

woman proposing to manOn leap day, tradition says it's OK for a woman to propose to a man. But then who gets the ring? (Photo: Antonio Guillem/Shutterstock)

According to tradition, it's OK for a woman to propose to a man on Feb. 29. The custom has been attributed to various historical figures including St. Bridget, who is said to have complained to St. Patrick that women had to wait too long for their suitor to pop the question. The obliging Patrick supposedly gave women one day to propose, says the BBC.

5. It's a day that doesn't legally exist

Another tale claims that Queen Margaret of Scotland (who would have been only 5 years old at the time, so take it with a grain of salt) enacted a law setting fines for men who turned down marriage proposals from women during a leap year. It's thought that the basis for the tradition likely goes back to the time when Feb. 29 wasn't recognized by English law; if the day had no legal status, it was OK to break with convention and a woman could propose.

6. But there may be a fine for not accepting

There are other traditions that put a price on saying "no." If a man doesn't accept a leap year proposal, it will cost him. In Denmark, a man refusing a woman's Feb. 29 proposal must give her a dozen pairs of gloves, according to The Mirror. In Finland, an uninterested gentleman must give his spurned suitor enough fabric to make a skirt.

7. It's bad for the marriage business

Not surprisingly, leap years can be bad for the nuptial business, too. One in five engaged couples in Greece avoid tying the knot in a leap year, reports The Telegraph. Why? Because they believe it's bad luck.

8. There's a Leap Year Capital

The twin cities of Anthony, Texas, and Anthony, New Mexico, are the self-proclaimed Leap Year Capital of the World. They hold a four-day leap year festival that includes a huge birthday party for all leap year babies. (ID required.)

9. About those leap year babies

four cupcakes with candlesWhen it's not a leap year, 'leaplings' have to celebrate on Feb. 28 or March 1. (Photo: Neirfy/Shutterstock)

People born on leap day are often called "leaplings" or "leapers." Most of them don't wait every four years to celebrate their birthdays, but instead blow out the candles on Feb. 28 or March 1. According to History.com, about 4.1 million people around the world have been born on Feb. 29, and the chances of having a leap birthday are one in 1,461.

10. Record-breaking babies

According to GuinnessWorld Records, the only verified example of a family producing three consecutive generations born on Feb. 29 belongs to the Keoghs. Peter Anthony Keogh was born in Ireland in 1940. His son, Peter Eric, was born in the U.K. on leap day in 1964, and his granddaughter Bethany Wealth was born in the U.K. in 1996. (We think that's kinda freaky.)

11. Famous people born on leap day

Famous people born on leap day include composer Gioacchino Rossini, motivational speaker Tony Robbins, jazz musician Jimmy Dorsey, actors Dennis Farina and Antonio Sabato Jr., and rapper/actor Ja Rule, to name a few.

12. Leap year proverbs

green frog The topic of frogs comes up a lot on Feb. 29, especially if you have kids and are looking for some fun activities. (Photo: Dave Young [CC BY 2.0]/Flickr)

There are lots of proverbs that revolve around leap year. In Scotland, leap year is thought to be bad for livestock, which is why the Scottish say, "Leap year was ne'er a good sheep year." In Italy, where they say "anno bisesto, anno funesto" (which means leap year, doom year), there are warnings against planning special activities such as weddings. The reason? "Anno bisesto tutte le donne senza sesto" which means "In a leap year, women are erratic."

13. There's even a leap year club

The Honor Society of Leap Year Day Babies is a club for people born on Feb. 29. More than 11,000 people worldwide are members. The goal of the group is to promote leap day awareness and to help leap day babies get in touch.

Editor's note: This story has been updated since it was published in February 2016.

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.

13 unexpected leap year facts
Feb. 29 is more than just a weird day to have a birthday. There are a lot of traditions guided by this almost-every-four-year surprise.