Pi, or π, is the ratio of the circumference of a circle to its diameter. It’s usually shortened to 3.14, which is why March 14 is celebrated every year as Pi Day, but it’s an irrational number that continues infinitely without a pattern, like so: 3.14159265358979323846264338327950288419716939937510582… The year 2015 marks a very special Pi Day, one that comes only once a century: 3.14.15. Hence some once-in-a-lifetime Pi Day celebrations and promotions around the country are planned for this Saturday. But what’s the big deal about pi in the first place? “To mathematicians, pi is a powerful superhero that turns up all the time in a huge range of mathematical problems,” Keith Devlin, Stanford University mathematician and the NPR “Math Guy”, explained recently to USA Today. “Pi deserves its special day.” Unsurprisingly, the are many Pi Day promotions that’ll get math nerds excited: For instance, the American Mathematical Society is discounting its publications (40% off for members, 25% off for nonmembers) for orders placed using the code MT159 from March 14 through the end of the month. It’s just like Black Friday! There are plenty of deals meant to appeal to C students who hated math too. The fast-casual Blaze Pizza chain is offering any pizza for $3.14 on Pi Day. Your Pie, a pizza restaurant franchise in the southeastern U.S., is knocking $3.14 off pizzas and handing out free gelato and pi-related swag on Saturday. Up in New England, Portland Pie Company locations host an annual customer appreciation day on Pi Day, and on 3.14.15 they will be selling $3.14 pints of beer, $3.14 kids meals, and $3.14 10-inch specialty pizza pies. Cities around the country have tons of Pi Day deals and events as well. Boston: In addition to restaurants in Boston and Cambridge offering special $3.14 pricing on pizza or pie on Saturday, institutions ranging from the Boston Children’s Museum to the Harvard University Math Department are hosting Pi Day activities. MIT, meanwhile, has made it tradition for 3.14 to be the day for admissions decisions to be announced. This year, the school created a goofy prank video, supposedly showing that students will be alerted of their acceptance via letters delivered by drones flown all over the world, with the dramatic “Flight of the Valkyries” playing in the background: Chicago: The day starts in the Windy City with a special 3.14-mile Pi K Fun Run/Walk in four locations, organized by the Illinois Science Council. To participate, tickets cost $31.41, though in homage to Albert Einstein you’ll get $5 off registration if your first name is Albert or Alberta. Later, the Adler Planetarium is hosting Ultimate Pi Day festivities on Saturday that include a pie eating contest, free samples of pizza and pie from local restaurants, and a nighttime event for those 21 and over with music, panel discussion, and a cash bar. Throughout the day, many bakeries and pizza places in the city are doing Pi Day promotions like $3.14 pies and $3.14 combos with a soda and a slice of pizza. New York City: MoMath, which is the hip name for the National Museum of Mathematics, is organizing a Pi Day of the Century nighttime event in Manhattan’s Madison Square Park. Participants will encircle a park fountain with glowing light sticks, and there will be a countdown to precisely 9:26 p.m., so that it will be 126.96.36.199.26 (just like pi), with free hot chocolate and pie to follow. The museum is hosting Pi Day activities earlier in the day for kids, starting with a countdown outside leading to 9:26 a.m. and an early 9:30 opening rather than the usual 10 a.m. Princeton, N.J.: Arguably no town celebrates Pi Day with as much brilliant enthusiasm as Princeton, where mathematical genius Albert Einstein—born on March 14 in 1879, by the way—lived and taught. Princeton is overloaded with Pi Day deals at restaurants, theaters, and shops along the lines of $3.14 off food orders and free slices of pie for customers. There are also special events scheduled throughout the day, including a parade (or “Pirade”), an Einstein lookalike contest, a pi recitation competition, and something dubbed the Nerd Herd Smart Phone Pub Crawl. San Francisco: The Exploratorium museum holds annual Pi Day activities, with pi- and pie-related panel discussions, movies, dough-tossing, and freebies on this year’s schedule. Washington, D.C.: The Washington Post rounded up a bunch of Pi Day deals and activities in the D.C. metro area, most involving $3.14 deals on pizza, pie, and in at least one case, beer.