March 14 (3/14) is celebrated annually as Pi Day because the date resembles the ratio of a circle’s circumference to its diameter — 3.14 for short. And this year's date syncs up with the first four digits after the decimal point, so 3.14.15 looks a lot like 3.14159265359 (etc). That won't happen again until 2115.
In case you don't remember from math class, Pi is an “irrational and transcendental number” with decimals that “continue infinitely without repetition or pattern,” according to PiDay.org, the official website for the holiday.
Celebrations are planned worldwide, but here is a glimpse at some of the most unusual ones. If you can't make any of them, plan your own around 9:26:53 in the morning or in the evening.
• Lucky high school seniors will find out at 9:26 a.m. if they got into the Massachusetts Institute of Technology in Cambridge, Mass. The letters could be delivered by drones, based on this video released by the admissions office.
• In New York City, hundreds are expected to gather around a park fountain at 9:26 a.m. and with glow sticks in hand at 9:26 p.m. (to represent pi as 3.1415926) in an event organized by the National Museum of Mathematics (MoMath).
• Lovebirds can get married — "pi the knot" — in Vegas for $314.15, a deal that comes with an apple pie.
• Chicago athletes and science lovers will run a "Pi K," organized by the Illinois Science Council. Because March 14 is also Albert Einstein's birthday, participants named "Albert, Alberta, Albertina or Alberts" got $5 off registration. A sternly-worded disclaimer says, "If you show up on the 14th and are not so appropriately named, you will be fined $3.14, have to run backwards the whole route, and people will call you "Einstein" but not in a positive way."
• Admission at the Salvador Dalí Museum in Saint Petersburg, Fla., will be $3.14. The surrealist often incorporated the mathematical principle into his art
• The National Security Agency's National Cryptographic Museum in Annapolis, Md., will show you how to measure the diameter of your head using pi.