- Pi Day is an annual celebration of the mathematical constant π (pi).
- Pi Day is observed on March 14 (3/14 in the month/day date format) since 3, 1, and 4 are the first three significant digits of π.
- On March 12, 2009, the U.S. House of Representatives passed a non-binding resolution (HRES 224), recognizing March 14, 2009 as National Pi Day.
- The earliest known official or large-scale celebration of Pi Day was organized by Larry Shaw in 1988 at the San Francisco Exploratorium, where Shaw worked as a physicist, with staff and public marching around one of its circular spaces, then consuming fruit pies.
- The entire month of March 2014 (3/14) was observed by some as "Pi Month".
- In the year 2015, Pi Day had special significance on 3/14/15 (mm/dd/yy date format) at 9:26:53 a.m. and also at p.m., with the date and time representing the first 10 digits of π.
- Pi Day of 2016 was also significant because its mm/dd/yy represents pi rounded to the first five digits.
- Pi Day has been observed in many ways, including eating pie, throwing pies and discussing the significance of the number π, due to a pun based on the words "pi" and "pie" being homophones in English (pronunciation: /paɪ/).
- Massachusetts Institute of Technology has often mailed its application decision letters to prospective students for delivery on Pi Day. Starting in 2012, MIT has announced it will post those decisions (privately) online on Pi Day at exactly 6:28 pm, which they have called "Tau Time", to honor the rival numbers pi and tau equally. In 2015, the regular decisions were put online at 9:26 AM, following that year's "pi moment".
- The town of Princeton, New Jersey, hosts numerous events in a combined celebration of Pi Day and Albert Einstein's birthday, which is also March 14. Einstein lived in Princeton for more than twenty years while working at the Institute for Advanced Study. In addition to pie eating and recitation contests, there is an annual Einstein look-alike contest.
- The number π is a mathematical constant, the ratio of a circle's circumference to its diameter, commonly approximated as 3.14159.
- It has been represented by the Greek letter "π" since the mid-18th century, though it is also sometimes spelled out as "pi" (/paɪ/).