Pi Day is an annual celebration of the mathematical constant 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 Pi. In 2009, the United States House of Representatives supported the designation of Pi Day.
Pi Approximation Day is observed on July 22 (22/7 in the day/month date format), since the fraction 22/7 is a common approximation of Pi, which is accurate to two decimal places and dates from Archimedes.
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 Exploratorium continues to hold Pi Day celebrations. On March 12, 2009, the U.S. House of Representatives passed a non-binding resolution (111 H. Res. 224), recognizing March 14, 2009 as National Pi Day. For Pi Day 2010, Google presented a Google Doodle celebrating the holiday, with the word Google laid over images of circles and pi symbols. 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 Pi. 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 Pi, due to a pun based on the words "pi" and "pie" being homophones in English.
Massachusetts Institute of Technology has often mailed its application decision letters to prospective students for delivery on Pi Day. Starting in 2012, MIT 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."
Princeton, NJ, 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.