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 digits of π. 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 format), since the fraction 22⁄7 is a common approximation of π, which is accurate to two decimal places and dates from Archimedes. Two Pi Day, also known as Tau Day, is lightly observed on June 28 (6/28 in the month/day format).

Pi Day | |
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Significance | 3, 1, and 4 are the three most significant figures of π in its decimal representation. |

Celebrations | Pie eating, discussions about π^{[1]} |

Date | March 14 |

Next time | March 14, 2019 |

Frequency | Annual |

First time | 1988 |

Related to | Pi Approximation Day |

Part of a series of articles on the |

mathematical constant π |
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3.1415926535897932384626433... |

Uses |

Properties |

Value |

People |

History |

In culture |

Related topics |

**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 digits of π.^{[2]}^{[3]} In 2009, the United States House of Representatives supported the designation of Pi Day.^{[4]}

**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.^{[5]}

**Two Pi Day**, also known as **Tau Day**, is lightly observed on June 28 (6/28 in the *month/day* format).^{[6]}

In 1988, the earliest known official or large-scale celebration of Pi Day was organized by Larry Shaw at the San Francisco Exploratorium,^{[7]} where Shaw worked as a physicist,^{[8]} with staff and public marching around one of its circular spaces, then consuming fruit pies.^{[9]} The Exploratorium continues to hold Pi Day celebrations.^{[10]}

On March 12, 2009, the U.S. House of Representatives passed a non-binding resolution (111 H. Res. 224),^{[4]} recognizing March 14, 2009 as National Pi Day.^{[11]} For Pi Day 2010, Google presented a Google Doodle celebrating the holiday, with the word Google laid over images of circles and pi symbols;^{[12]} and for the 30th anniversary in 2018, it was a Dominique Ansel pie with the circumference divided by its diameter.^{[13]}

The entire month of March 2014 (3/14) was observed by some as "Pi Month".^{[14]}^{[15]} In the year 2015, March 14 was celebrated as "Super Pi Day".^{[16]} 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.^{[17]}

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 ( /paɪ/), and the coincidental circular nature of a pie.^{[1]}^{[18]}

Massachusetts Institute of Technology has often mailed its application decision letters to prospective students for delivery on Pi Day.^{[19]} 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.^{[20]}^{[21]} In 2015, the regular decisions were put online at 9:26 AM, following that year's "pi minute".^{[22]} 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.^{[23]}^{[24]} Celebrations of this date jokingly suggest eating "twice the pie".^{[25]}^{[26]}^{[27]}

Princeton, New Jersey, hosts numerous events in a combined celebration of Pi Day and Albert Einstein's birthday, which is also March 14.^{[28]} 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.^{[28]}

- Pi Pie at Delft University
- Blackberry Pi Pie
- A grocery store selling pies for 3.14 dollars, Pi Day 2015

- 1 2 Landau, Elizabeth (March 12, 2010). "On Pi Day, one number 'reeks of mystery'". CNN. Retrieved March 14, 2018.
- ↑ Bellos, Alex (March 14, 2015). "Pi Day 2015: a sweet treat for maths fans".
*theguardian.com*. Retrieved March 14, 2016. - ↑ Program on Sveriges Radio – Swedish national radio company Read March 14, 2015
- 1 2 United States. Cong. House. Supporting the designation of Pi Day, and for other purposes. 111th Cong. Library of Congress.
- ↑ "Pi Approximation Day is celebrated today".
*Today In History*. Verizon Foundation. Retrieved January 30, 2011. - ↑ Tau Day: Why you should eat twice the pie – Light Years – CNN.com Blogs Archived January 12, 2013, at the Wayback Machine
- ↑ Berton, Justin (March 11, 2009). "Any way you slice it, pi's transcendental".
*San Francisco Chronicle*. Retrieved March 18, 2011. - ↑ Borwein, Jonathan (March 10, 2011). "The infinite appeal of pi". Australian Broadcasting Corporation. Retrieved March 13, 2011.
- ↑ Apollo, Adrian (March 10, 2007). "A place where learning pi is a piece of cake" (PDF). The Fresno Bee.
- ↑ "Exploratorium 22nd Annual Pi Day". Exploratorium. Retrieved January 31, 2011.
- ↑ McCullagh, Declan (March 11, 2009). "National Pi Day? Congress makes it official".
*Politics and Law*. CNET News. Retrieved March 14, 2009. - ↑ "Pi Day".
*Google Doodles*. Google. Retrieved October 9, 2012. - ↑ "30th Anniversary of Pi Day!".
*www.google.com*. Retrieved March 19, 2018. - ↑ Main, Douglas (March 14, 2014). "It's Not Just Pi Day, It's Pi Month!". Popular Science. Retrieved July 22, 2014.
- ↑ "Pi Month Celebration & Circle of Discovery Award Presentation | College of Computer, Mathematical, and Natural Sciences". Cmns.umd.edu. March 11, 2014. Retrieved July 22, 2014.
- ↑ https://www.forbes.com/sites/ericmack/2015/03/14/happy-super-pi-day-heres-how-to-celebrate/
- ↑ Ro, Sam (March 13, 2014). "March 14, 2015 Will Be A Once-In-A-Century Thrill For Math Geeks".
*Business Insider*. Retrieved March 13, 2014. - ↑ Smith, K.N. "Wednesday's Google Doodle Celebrates Pi Day".
- ↑ McClan, Erin (March 14, 2007). "Pi fans meet March 14 (3.14, get it?)". msnbc.com. Retrieved January 24, 2008.
- ↑ "I have SMASHING news!". MIT Admissions. Retrieved March 12, 2012.
- ↑ McGann, Matt. "Pi Day, Tau Time". MIT Admissions. Retrieved March 18, 2012.
- ↑ "Keep your eyes to the skies this Pi Day". MIT Admissions. Retrieved March 14, 2018.
- ↑ "It's Pi Day today. But these people say we should refuse to celebrate it".
*The Independent*. March 13, 2018. Retrieved March 14, 2018. - ↑ "Pi Day Turns 25: Why We Celebrate an Irrational Number". March 14, 2013. Retrieved March 14, 2018.
- ↑ Bartholomew, Randyn Charles. "Why Tau Trumps Pi".
*Scientific American*. - ↑ Landau, Elizabeth. "In case Pi Day wasn't enough, it's now 'Tau Day' on the Internet".
*CNN*. - ↑ "Tau Day – Come Eat Twice the (Pi)e".
- 1 2 "Princeton Pi Day & Einstein Birthday Party".
*Princeton Regional Convention and Visitors Bureau*. Retrieved February 9, 2019.

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