Nearly every four years, the Gregorian calendar — which is used in the majority of countries around the world — gets an extra day: February 29.

For some people, leap day means frog jokes and extravagant birthday parties. For many, it may conjure memories of the 2010 rom-com Leap Year, which harkens back to the Irish tradition by which women can propose to men on that one day. And others likely see it merely as a funny quirk in the calendar, or just another Thursday.

Leap day means several different things to Alexander Boxer, a data scientist and the author of A Scheme of Heaven: The History of Astrology and the Search for Our Destiny in Data.

Literally speaking, he says, it's an "awkward calendar hack" aimed at making up for the fact that a year isn't a flat number of days, but more like 365 and a quarter. But there's more to it than that.

"I think the significance of the leap year is that it's a great reminder that the universe is really good at defying our attempts to devise nice and pretty and aesthetically pleasing systems to fit it in," he told NPR's Morning Edition.

Boxer says it's also a great reminder that the calendar most people rely on every day is actually the product of multiple civilizations, building off each other as they share in what he calls "this great undertaking of trying to understand time."

So where did leap year come from, and what are we supposed to do with our extra day? NPR's Morning Edition spoke with experts in astronomy, history and economics to find out.

Why do we have leap years?

Most people know that a single day is about 24 hours long, and that there are 365 days in a year.

But it actually takes Earth 365.242190 days to orbit the sun, says Jackie Faherty, an astronomer at the American Museum of Natural History in New York.

"And that .242190 days to go around the sun is the entire reason why we have a leap year," she explained.

Centuries ago, people kept track of the sun's position — such as for a solstice or the longest day of the year — to know when to do things like plant and harvest. Over time, she says, the need grew for a centralized calendar system.

The Hebrew, Chinese and Buddhist calendars, among others, have long contained entire leap months. The West is no stranger to leap years either.

The Julian Calendar, which Julius Caesar introduced in 45 BC, included an extra day every year. He borrowed the idea from the Egyptians, though his math wasn't exactly correct. Caesar overestimated the solar year by about 11 minutes, leading to an overcorrection by about eight days each millennium. That explains why Easter, for example, fell further and further away from the spring equinox over time.

Pope Gregory XIII sought to address that problem in the 16th century with the Gregorian Calendar, which adds leap days in years divisible by four, unless the year is also divisible by 100. To make matters even more confusing, a leap day is still added in years divisible by 400.

Why add the extra day in February? Boxer, the data scientist, says the Romans considered it an unlucky month. On top of that, they were deeply suspicious of odd numbers. Because February only had 28 days to begin with, they "just shoved it into February," though leap day used to be on the 24th.

Ultimately, says Boxer, the calendar is a compromise.

"On the one hand, you don't want a calendar that makes it so complicated to know how many days it's going to be from one year to the next," he added. "But on the other hand, you want to make sure that winter holidays, too, in the winter and summer holidays, stay in the summer, especially if your holidays are related to things like agriculture, harvest holidays and whatnot."

What does leap day mean for birthdays?

One tangible impact of a leap year is that birthdays will fall on a different day of the week than their usual pattern.

"If your birthday was on a Tuesday last year, you're going to skip over Wednesday and you'll have a birthday on a Thursday," said Faherty. "Not to mention those poor people that are born on February 29, a day that only exists every four years."

There are about 5 million people worldwide with a Feb. 29 birthday, according to the History Channel. The list of so-called "leaplings" includes celebrities such as motivational speaker Tony Robbins and hip-hop artist Ja Rule. And peoples' odds of joining their ranks are small — about 1-in-1,461, to be exact.

Several leaplings told NPR that there's no set rule on which day to celebrate their birthday in a non-leap year. Some prefer Feb. 28, others March 1 and many do both.

"My answer to this question has evolved over the years," said Michael Kozlowski Jr., a leap day baby based in Belgium. "It used to be February for the reasons that I identified more with that month compared to March. But these days I honestly like to celebrate both days or even the entire week. It seems only fair and it works and it feels great."

They acknowledged both pros and cons of having a leap day birthday. Several said that while they were teased about it in grade school, it helped them develop a thicker skin and gave them a fun fact for life — plus more days to celebrate.

Plus, many online forms — including for the DMV — don't recognize Feb. 29 as a possible birth date. Raenell Dawn, the co-founder of the Honor Society of Leap Year Day Babies, told NPR in 2020 that those logistics can cause trouble, especially when it comes to things like driver's license expirations. But she also said there's no reason for leaplings to change their birth date.

"Humans program the computer, so the humans need to program it correctly," she said. "'Cause February 29 is everyone's extra day. And it's a day that started in 45 B.C. And it's the most important date on the calendar because it keeps all the dates on the calendar in line with the seasons."

What should you do — and not do — on Feb. 29?

There are lots of superstitions and traditions about leap day on the internet, and a few celebrations to look forward to IRL.

A decades-old French satirical newspaper, La Bougie du Sapeur, goes to print only on Feb. 29 — this year included. There are also festivities in the "Leap Year Capital of the World," as Anthony, Texas, is known.

Leapling Mary Ann Brown petitioned Congress to give Anthony, Texas — and Anthony, New Mexico, on the other side of the state line — that designation in 1988 because of the "numerous number of leap year births that happened within the two towns," Mayor Anthony Turner told NPR over email.

In years past, he said, the community marked leap day with a parade that stretched between the two towns of Anthony. This year, the Texas side is hosting a two-day leap year festival, complete with live music, local vendors and an exclusive barbecue dinner for leap day babies.

"This is an opportunity for the community to take pride in the fact that they live in the leap year capital of the world, and a great chance for everyone from everywhere to join us and enjoy the true beauty of our lovely town," Turner added.

Worldwide, most leap day lore revolves around romance and marriage, as the History Channel explains.

According to one legend, complaints from St. Bridget prompted St. Patrick to designate Feb. 29 as the one day when women can propose to men. The custom spread to Scotland and England, where the British said that any man who rejects a woman's proposal owes her several pairs of fine gloves. In Greece and some other places, it's considered bad luck to get married on leap day.

Katherine Parkin, a history professor at Monmouth University, said she doesn't believe any of the myths are true — but doesn't think they had to be in order to take hold, which they did in America as early as the 1780s.

The real origin, she believes, is that people have historically liked to challenge gender and gender roles.

"And in the case of marriage, to have a reversal of that power, I think is really unusual," she added. "And it ties perfectly with this unusual date. Where did it come from and where did it go? And so I think it really plays well into people's imagination and playfulness."

But Parkin says her research points to darker undertones behind the tradition — namely, that it was actually intended to ridicule women.

She points to the proliferation of postcards in the 20th century — which people would send each other across all kinds of relationships — that portrayed women who proposed to men as desperate, unattractive and aggressive, such as holding butterfly nets and pointing guns at guys.

"It's proving to ... reinforce that it's a leap year and that this tradition exists and yet at the same time telling women, you really don't want to do this because it looks bad for you," Parkin said. "As a historian, I look back to this tradition and see it as part of an American desire to offer women false empowerment."

Of the more than 100 people who responded to an NPR callout about their leap day celebrations and traditions, several said they had gotten engaged or married on Feb. 29. Only one explicitly mentioned gender roles.

"I think this is the day that (traditionally) a woman was able to propose?" wrote Suzanne Forbes. "If so, I plan on proposing to myself in a beautiful southern setting (likely [Georgia], while solo kayaking)!"

What if we didn't have leap years?

Not everyone loves leap day.

Steve Hanke, a professor of applied economics at Johns Hopkins University, is one critic. He argues that the current calendar, in which dates occur on different days of the week each year, creates scheduling problems as well as confusion around holiday dates.

That's why he and Johns Hopkins astrophysics professor Dick Henry have created the Hanke–Henry Permanent Calendar, a proposal for a new calendar that would implement an occasional leap week rather than leap day.

"The great thing about the permanent calendar is that it never changes," Hanke explained. "The date would be on the same day. Every year, year after year after year ... January 1st is always on a Monday. July 4th is always on a Thursday. December 25th, Christmas, is always on a Monday."

Their calendar divides the year into four three-month quarters, each with the same number of days. The first two months of each quarter — including January and February — would always have 30 days, and the third month would have 31. Every six years, there would be an extra seven days at the end of December, which Hanke says would "eliminate calendar drift."

Hanke argues that his proposed calendar would save confusion and potentially money, pointing to studies in the United Kingdom that show the economic gains associated with having public holidays on weekends. And he believes it would be easy for a president to implement the new system by executive order, something that he and Henry have even drafted, just in case.

Still, he describes their lobbying efforts as more of a "soft sell" at the moment.

It seems like the current calendar system — with its leap days and years — may be here to stay, despite the many possible alternates. Faherty, the astronomer, says if someone truly wanted to keep track of their path around the sun, one could "build yourself a henge and know when the solstice is and carry on from that."

"But we don't do that," she said. "We gave it an interval and we follow that, so now we're stuck. And now you have to enter these leap days, to try and do our best to fix the human need to have a document that says where exactly you are in the position that the Earth is falling around."

And that's probably enough to think about for the year, maybe even the next three.

Adam Bearne and Julie Depenbrock contributed reporting.