|Observed by||United States|
|Significance||3, 1, and 4 are the three most significant figures of π in its decimal representation.|
|Celebrations||Pie eating, pi memorization competitions, discussions about π|
|Next time||March 14, 2023|
|Related to||Pi Approximation Day|
|Part of a series of articles on the|
|mathematical constant π|
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 format) since 3, 1, and 4 are the first three significant figures of π. It was founded in 1988 by Larry Shaw, an employee of the San Francisco, California science museum, the Exploratorium. Celebrations often involve eating pie or holding pi recitation competitions. In 2009, the United States House of Representatives supported the designation of Pi Day. UNESCO's 40th General Conference designated Pi Day as the International Day of Mathematics in November 2019. Alternative dates for the holiday include July 22[alpha 1] (22/7, an approximation of π) and June 28 (6.28, an approximation of 2π or tau).
In 1988, the earliest known official or large-scale celebration of Pi Day was organized by Larry Shaw 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; and for the 30th anniversary in 2018, it was a Dominique Ansel pie with the circumference divided by its diameter.
The entire month of March 2014 (3/14) was observed by some as "Pi Month". In the year 2015, March 14 was celebrated as "Super Pi Day". It had special significance, as the date is written as 3/14/15 in month/day/year format. At 9:26:53, the date and time together represented the first 10 digits of π, and later that second Pi Instant represented all of π's 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 ( //), and the coincidental circular shape of many pies. Many pizza and pie restaurants offer discounts, deals, and free products on Pi Day. Also, some schools hold competitions as to which student can recall pi to the highest number of decimal places.
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 minute", and in 2020, regular decisions were released at 1:59 pm, making the first six digits of pi.
June 28 is "Two Pi Day", also known as "Tau Day". 2π, also known by the Greek letter tau (𝜏) is a common multiple in mathematical formulae. Some have argued that τ is the more fundamental constant, and that Tau Day should be celebrated instead. Celebrations of this date jokingly suggest eating "twice the pie".
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.
Pi Pie at Delft University
Pi Day is most frequently observed on March 14, but related celebrations have been held on alternative dates.
Pi Approximation Day is observed on July 22 (22/7 in the day/month format), since the fraction 22⁄7 is a common approximation of π, which is accurate to two decimal places and dates from Archimedes.
Pi Approximation Day is also observed on the 10th of November since it is the 314th day of the year. 
- Mole Day
- Sequential time
- Square Root Day
- White Day, also on March 14th
- List of minor secular observances
- 22 July in the day/month format
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