Maggie Donahue, 33, and Mike Karmel, 32, both of Petworth, are getting married at the Carnegie Institution for Science on Pi Day. (Courtesy Maggie Donahue)

The bride and groom were rushing to finish everything before the big day, picking up rings and dresses, calling the minister, cleaning the house, checking in with the DJ to decide what song will play when they cut the pie.

Or rather, the Π.

When you’re getting married on Pi Day, there must be pie. Maggie Donahue and Mike Karmel will tie the knot on Saturday at the Carnegie Institution for Science with cherry pie, apple pie, key lime pie and chocolate bourbon pecan pie.

They had to go all out, they say, for “the big day” — the big Pi Day, when the date, March 14, doesn’t just align with 3.14. Once in a century, the numbers in the year bring in two more digits of the mathematical constant to make 3.1415. It’s happening this weekend, so plan accordingly.

Quick refresher: Pi is what you get when you divide the circumference of a circle, any circle, by its diameter. It’s always 3.1415926 . . . followed by an infinite stream of digits that never repeat, as far as we know. (And computers have calculated trillions of digits of pi, so we’re pretty sure.)

The mysterious number has been a subject of fascination for as long as numbers have been studied.

“Pi is sort of a celebrity among the numbers,” said Brady Haran, a BBC journalist turned vlogger known for his popular YouTube channel, Numberphile. “Because of a quirk of fate, pi happened to represent this ratio for all circles, so it was swept up from obscurity, thrust into the limelight and given its hero status.”

Among self-proclaimed geeks, pi is a bit of a pride point, one of the few mathematical symbols widely recognized in popular culture. There was pi in an episode of “The Simpsons” and an episode of “Star Trek.” Π is the secret code in Alfred Hitchcock’s “Torn Curtain” and Sandra Bullock’s “The Net.” Givenchy has a Π cologne.

“When someone says, ‘Hey, happy Pi Day!’ You can be like, ‘Happy Pi Day!’ And you just think, maybe they weren’t a person you were expecting to be into Pi Day, but then you find out they are,” said Karmel, 32, the half of the couple who wanted to get married on Pi Day.

Actually, he wanted to get married on May 4, Star Wars Day (as in, “May the 4th be with you”) but it won’t fall on a Saturday until 2019. He checked.

Donahue, 33, a Legal Aid attorney, fell for Karmel’s geeky side as soon as they met. On a beach vacation with her friends in 2012, a psychic told her, “You are about to meet your soulmate.” Three days later, she walked into Kostume Karaoke night at Solly’s Tavern along the U Street corridor and saw a man onstage croaking out the Backstreet Boys’s “I Want It That Way.” By the end of the night, he would be serenading her with Cake’s “The Distance” — the song the DJ will play when they cut the pie.

They met each other’s friends, met each other’s parents, moved in together, all the while maintaining the quirkiness that their relationship began with. Their dog is named “Maeby,” after a character in “Arrested Development.” There are Star Wars legos and lightsabers lying around the house. Science fiction shows are regularly on TV. A Pi Day-themed wedding seemed just right.

The allure of Pi Day for Karmel, a federal contractor, is the same reason Pi Day is good for teachers: It’s kind of fun. Circles are everywhere, meaning pi is everywhere, presenting endless opportunities for engaging students.

Pi is part of history. An estimation of pi is found in the Old Testament, where King Solomon’s pool is measured at 10 cubits across and 30 cubits around.

It inspires word games: A pi poem, or “piem” can be made by arranging words with the number of letters in 3.14159265, such as “See I have a rhyme assisting my feeble brain.”

And maybe even philosophy.

“The most interesting thing about pi is that it seems to represent the unknowable,” said David Blatner, author of “The Joy of Pi.” “We can calculate almost anything, but when it comes to a simple circle, we literally cannot compute the distance around the circle from its diameter.”

Plus, there’s the edible kind of Π. The obvious pun is Donahue’s favorite part of the day, and the reason she agreed to get married on March 14. In her undergrad years at Colgate University, she helped found an organization called “Eta Bita Pi.” It was a “sorority” her mom had joined in college, and when her friends heard about it, they wanted to be an Eta Bita Pi, too. To the likely chagrin of the campus’s actual Greek community, Eta Bita Pi became one of the largest student groups on campus by the time Donahue graduated, she says.

“So I feel like I have claim to the day, too,” she said.

People joined Eta Bita Pi mostly for the T-shirts, Donahue said. They meant to organize a pie-eating contest but never got around to it.

Pi Day around Washington is all about the contests, and the pies. Pizza restaurant District of Pi will run a pi memorization competition. Dangerously Delicious Pies, Baked & Wired, 2941 and Teeny Pies are selling discounted pies, some even with a Π on them. In Georgetown, you can get a beer for $3.14 at Pizzeria Paradiso.

Donahue and Karmel, meanwhile, will be on their way down the aisle.

“We thought about doing something at 9:26 in the morning,” Karmel said. Get it? The first eight digits of pi, 3/14/15 9:26.

Donahue rolled her eyes.

“I’m pretty sure I’ll be sleeping,” she said.