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{{Short description|Annual mathematical celebration on March 14}}
{{Distinguish|National Pie Day}}
{{Use American English|date=July 2022}}
{{Use mdy dates|date=March 2024}}
{{Infobox holiday
| holiday_name  = Pi Day
| image         = Larry Shaw, the founder of Pi Day, at the Exploratorium in San Francisco--2007 March 21.jpg
| caption       = [[Larry Shaw (physicist)|Larry Shaw]], the organizer of the first Pi Day celebration, at the [[Exploratorium]] in [[San Francisco]]
| observedby    = United States
| significance  = 3, 1, and 4 are the three most [[significant figure]]s of {{pi}} in its decimal representation.
| duration      = 1 day
| frequency     = Annual
| scheduling    = same day each year
| date          = March 14
| mdy           = y
| firsttime     = 1988
| celebrations  = [[Pie]] eating, [[pi memorization]] competitions, discussions about {{pi}}<ref name="landau">{{Cite news |last=Landau |first=Elizabeth |author-link=Elizabeth Landau |date=March 12, 2010 |title=On Pi Day, one number 'reeks of mystery' |language=en |publisher=CNN |url=http://www.cnn.com/2010/TECH/03/12/pi.day.math/index.html |access-date=March 14, 2018}}</ref>
| relatedto     = Pi Approximation Day
| type          = default
| longtype      = Mathematical
}}
{{Pi box}}
'''Pi Day''' is an [[list of minor secular observances#March|annual celebration]] of the [[mathematical constant]] [[pi|{{pi}} (pi)]]. Pi Day is observed on [[March 14]] (the 3rd month) since 3, 1, and 4 are the first three [[significant figures]] of {{pi}}, and was first celebrated in the United States.<ref>{{Cite web |last=Bellos |first=Alex |date=March 14, 2015 |title=Pi Day 2015: a sweet treat for maths fans |url=https://www.theguardian.com/science/2015/mar/13/pi-day-celebration-maths-fans-language-memory-contests |access-date=March 14, 2016 |website=[[The Guardian]]}}</ref><ref>{{Cite web |date=March 14, 2015 |title=Nedräkning mot internationella Pi-dagen |language=sv |url=http://sverigesradio.se/sida/avsnitt/514543?programid=412 |website=Swedish national radio company}}</ref> It was founded in 1988 by [[Larry Shaw (physicist)|Larry Shaw]], an employee of a [[science museum]] in [[San Francisco]], the [[Exploratorium]]. Celebrations often involve eating [[pie]] or holding [[Piphilology|pi recitation]] competitions. In 2009, the [[United States House of Representatives]] supported the designation of Pi Day.<ref name="HORRes">United States. Cong. House. Supporting the designation of Pi Day, and for other purposes. 111th Cong. [http://thomas.loc.gov/cgi-bin/query/z?c111:H.RES.224: Library of Congress] {{Webarchive|url=https://web.archive.org/web/20090807131531/http://thomas.loc.gov/cgi-bin/query/z?c111:H.RES.224: |date=August 7, 2009 }}</ref> [[UNESCO]]'s 40th General Conference designated Pi Day as the International Day of Mathematics in November 2019.<ref>{{Cite web |date=March 4, 2020 |title=International Day of Mathematics |url=https://en.unesco.org/commemorations/mathematics |website=UNESCO |language=en}}</ref><ref>{{Cite journal |last=Rousseau |first=Christiane |url=https://www.ams.org/journals/notices/201908/rnoti-p1306.pdf |title=International Day of Mathematics |date=September 1, 2019 |journal=Notices of the American Mathematical Society |volume=66 |page=1 |doi=10.1090/noti1928 |doi-access=free |issue=8}}</ref>

Other dates when people celebrate pi include Pi Approximation Day on July 22 (22/7 in the ''day/month'' format), a closer [[Approximations of π|approximation of {{pi}}]]; and June 28 (6.28), an approximation of 2{{pi}} or [[Tau (mathematical constant)|{{tau}}]] (tau).

== History ==
In 1988, the earliest known official or large-scale celebration of Pi Day was organized by [[Larry Shaw (physicist)|Larry Shaw]] at the [[Exploratorium|San Francisco Exploratorium]],<ref>{{Cite web |last=Berton |first=Justin |date=March 11, 2009 |title=Any way you slice it, pi's transcendental |url=https://www.sfgate.com/science/article/Any-way-you-slice-it-pi-s-transcendental-3169091.php |access-date=March 18, 2011 |website=[[San Francisco Chronicle]]}}</ref> where Shaw worked as a [[physicist]],<ref name="ABC – Science – 10 Mar 2011 – infinite appeal of pi">{{Cite web |last=Borwein |first=Jonathan |author-link=Jonathan Borwein |date=March 10, 2011 |title=The infinite appeal of pi |url=http://www.abc.net.au/science/articles/2011/03/10/3158045.htm?site=science/opinion |access-date=March 13, 2011 |publisher=[[Australian Broadcasting Corporation]]}}</ref> with staff and public marching around one of its circular spaces, then consuming fruit pies.<ref name="fresnobee">{{Cite news |last=Apollo |first=Adrian |date=March 10, 2007 |title=A place where learning pi is a piece of cake |work=[[The Fresno Bee]] |url=http://apollotutoring.com/pi_day.pdf}}</ref> The Exploratorium continues to hold Pi Day celebrations.<ref name="exploratorium">{{Cite web |title=Exploratorium 22nd Annual Pi Day |url=http://www.exploratorium.edu/pi/index.html |url-status=dead |archive-url=https://web.archive.org/web/20110314012746/http://www.exploratorium.edu/pi/index.html |archive-date=March 14, 2011 |access-date=January 31, 2011 |publisher=Exploratorium}}</ref>

On March 12, 2009, the U.S. House of Representatives passed a [[non-binding resolution]] ([[wikisource:H. RES. 224 Supporting the designation of Pi Day, and for other purposes|111 H. Res. 224]]),<ref name="HORRes" /> recognizing March 14, 2009, as National Pi Day.<ref>{{Cite news |last=McCullagh |first=Declan |date=March 11, 2009 |title=National Pi Day? Congress makes it official |work=Politics and Law |publisher=[[CNET News]] |url=http://news.cnet.com/8301-13578_3-10194354-38.html?part=rss&subj=news&tag=2547-1_3-0-5 |access-date=March 14, 2009}}</ref> For Pi Day 2010, Google presented a [[Google Doodle]] celebrating the holiday, with the word Google laid over images of circles and pi symbols;<ref name="GoogleDoodle">{{Cite web |title=Pi Day |url=https://doodles.google/doodle/pi-day/ |access-date=October 9, 2012 |website=Google Doodles}}</ref> and for the 30th anniversary in 2018, it was a [[Dominique Ansel]] pie with the [[circumference]] divided by its [[diameter]].<ref>{{Cite web |title=30th Anniversary of Pi Day! |url=https://doodles.google/doodle/30th-anniversary-of-pi-day/ |access-date=March 19, 2018 |website=www.google.com |language=en}}</ref>

Some observed the entire month of March 2014 (3/14) as "Pi Month".<ref>{{Cite magazine |last=Main |first=Douglas |date=March 14, 2014 |title=It's Not Just Pi Day, It's Pi Month! |url=http://www.popsci.com/article/science/its-not-just-pi-day-its-pi-month |magazine=Popular Science |access-date=July 22, 2014}}</ref><ref>{{Cite web |date=March 11, 2014 |title=Pi Month Celebration & Circle of Discovery Award Presentation &#124; College of Computer, Mathematical, and Natural Sciences |work=College of Computer, Mathematical, and Natural Sciences |url=http://cmns.umd.edu/news-events/events/1774 |access-date=July 22, 2014 |publisher=Cmns.umd.edu |author1=Jencasa }}</ref> In the year 2015, March 14 was celebrated as "Super Pi Day".<ref>{{Cite web |last=Mack |first=Eric |date=March 14, 2015 |title=Celebrate The Only Super Pi Day Of The Century |url=https://www.forbes.com/sites/ericmack/2015/03/14/happy-super-pi-day-heres-how-to-celebrate/ |access-date=March 14, 2019 |website=Forbes |language=en}}</ref> 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 ten digits of {{pi}},<ref>{{Cite web |last=Ro |first=Sam |date=March 13, 2014 |title=March 14, 2015 Will Be A Once-In-A-Century Thrill For Math Geeks |url=http://www.businessinsider.com/pi-day-2015-2014-3 |access-date=March 13, 2014 |website=Business Insider}}</ref> and later that second, "Pi Instant" represented all of {{pi}}'s digits.<ref>{{Cite journal |last=Rosenthal |first=Jeffrey S. |url=http://probability.ca/jeff/writing/PiInstant.html |title=Pi Instant |date=February 2015 |journal=Math Horizons |volume=22 |page=22 |doi=10.4169/mathhorizons.22.3.22 |issue=3 |s2cid=218542599|url-access=subscription }}</ref>

== Observance ==
[[Image:Happy Pi Day sign for sales on pies at Delicious Orchards.jpg|thumb|left|upright|Pi Day related offering at [[Delicious Orchards]], a country food market in [[Colts Neck Township, New Jersey|Colts Neck, New Jersey]]]]
Pi Day has been observed in many ways, including eating [[pie]], [[pieing|throwing pies]] and discussing the significance of the number {{pi}}.<ref>{{cite news|url=https://apnews.com/article/pi-day-celebration-math-infinite-number-f426c8659529cb07655b5f07b99ce1dd|title=What's Pi Day all about? Math, science, pies and more|first=Curt|last=Anderson|work=[[Associated Press]]|date=March 14, 2024|access-date=March 14, 2025|archive-url=https://web.archive.org/web/20240314074455/https://apnews.com/article/pi-day-celebration-math-infinite-number-f426c8659529cb07655b5f07b99ce1dd|archive-date=March 14, 2024|url-status=live}}</ref> The first two are due to a [[pun]] based on the words "pi" and "pie" being [[homophone]]s in English ({{IPAc-en|pron|p|aɪ}}), and the coincidental circular shape of many pies.<ref name="landau" /><ref>{{Cite web |last=Smith |first=K.N. |title=Wednesday's Google Doodle Celebrates Pi Day |url=https://www.forbes.com/sites/kionasmith/2018/03/14/wednesdays-google-doodle-celebrates-pi-day/ |website=[[Forbes]]}}</ref> Many [[pizza]] and pie restaurants offer discounts, deals, and free products on Pi Day.<ref name="nbcdfw">{{Cite news |last=<!--Not stated--> |date=March 13, 2021 |title=Celebrate Pi Day With These Deals Around North Texas |work=NBC DFW |url=https://www.nbcdfw.com/news/local/celebrate-pi-day-with-these-deals-around-north-texas/2578077/ |access-date=May 14, 2022}}</ref> Also, some schools hold competitions as to which student can [[Piphilology|recall pi to the highest number of decimal places]].<ref>{{Cite web |title=Honiton Community College Pi Day – Jazmin Year 9 |url=http://www.youtube.com/watch?v=dfM1i9_Un_Y) |url-status=dead |archive-url=https://web.archive.org/web/20131017125801/http://www.youtube.com/watch?v=dfM1i9_Un_Y |archive-date=October 17, 2013 |access-date=July 22, 2013 |publisher=YouTube}}</ref><ref>{{Cite web |title=HCC Celebrate International Pi Day |url=http://www.honitoncollege.devon.sch.uk/index.php/about-honiton-community-college/10-honiton-community-college-news/146-hcc-international-pi-day-champions |url-status=dead |archive-url=https://web.archive.org/web/20140222033538/http://www.honitoncollege.devon.sch.uk/index.php/about-honiton-community-college/10-honiton-community-college-news/146-hcc-international-pi-day-champions |archive-date=February 22, 2014 |access-date=July 22, 2013 |publisher=Honitoncollege.devon.sch.uk}}</ref>

The [[Massachusetts Institute of Technology]] has often mailed its [[College admissions in the United States#Application process|application decision letters]] to prospective students for delivery on Pi Day.<ref>{{Cite news |last=McClan |first=Erin |date=March 14, 2007 |title=Pi fans meet March 14 (3.14, get it?) |publisher=[[NBC News]] |url=http://www.nbcnews.com/id/17605924 |archive-url=https://web.archive.org/web/20130927144029/http://www.nbcnews.com/id/17605924/ |url-status=dead |archive-date=September 27, 2013 |access-date=January 24, 2008}}</ref> Starting in 2012, MIT has announced it will post those decisions (privately) online on Pi Day at exactly 6:28&nbsp;pm, which they have called "Tau Time", to honor the rival numbers {{pi}} and {{tau}} equally.<ref>{{Cite web |title=I have SMASHING news! |date=March 7, 2012 |url=http://mitadmissions.org/blogs/entry/i-have-smashing-news |access-date=March 12, 2012 |publisher=MIT Admissions}}</ref><ref>{{Cite web |last=McGann |first=Matt |title=Pi Day, Tau Time |date=March 13, 2012 |url=http://mitadmissions.org/blogs/entry/pi-day-tau-time |access-date=March 18, 2012 |publisher=MIT Admissions}}</ref> In 2015, the regular decisions were put online at 9:26&nbsp;am, following that year's "pi minute",<ref>{{Cite web |title=Keep your eyes to the skies this Pi Day |date=March 6, 2015 |url=http://mitadmissions.org/blogs/entry/keep-your-eyes-to-the-skies |access-date=March 14, 2018 |publisher=MIT Admissions |language=en}}</ref> and in 2020, regular decisions were released at 1:59 pm, making the first six digits of pi.<ref>{{Cite web |title=[Pinned] This is the way...to check your decisions |url=https://mitadmissions.org/blogs/entry/this-is-the-way-to-check-your-decisions/ |access-date=March 14, 2020 |website=MIT Admissions |date=March 10, 2020 |language=en-US}}</ref>

[[Princeton, New Jersey]], hosts numerous events in a combined celebration of Pi Day and [[Albert Einstein]]'s birthday, which is also March 14.<ref name="princeton">{{Cite web |title=Princeton Pi Day & Einstein Birthday Party |url=https://www.visitprinceton.org/events/annual-events/pi-day/ |access-date=February 9, 2019 |website=Princeton Regional Convention and Visitors Bureau}}</ref> 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.<ref name="princeton" />

In 2024, the [[recreational mathematics|recreational mathematician]] [[Matt Parker]] and a team of hundreds of volunteers at [[City of London School]] spent six days calculating 139 correct digits of pi by hand, in what Parker claimed was "the biggest hand calculation in a century".<ref name="Piday2024">{{cite AV media |people=Matt Parker |date=13 March 2024 |title=The biggest hand calculation in a century! [Pi Day 2024] |type=video |url=https://www.youtube.com/watch?v=LIg-6glbLkU |access-date=1 July 2024 |via=YouTube |location=London}}</ref><ref>{{Citation|url=https://tomrocksmaths.com/2024/04/04/carnival-of-mathematics-226/|title=Carnival of Mathematics 226|first=Tom|last=Crawford|publisher=The Aperiodical|date=4 April 2024|access-date=30 August 2024|archive-url=https://web.archive.org/web/20240830074418/https://tomrocksmaths.com/2024/04/04/carnival-of-mathematics-226/|archive-date=30 August 2024|url-status=live}}</ref> On 15 August 2024, the [[main-belt asteroid]] [[List of minor planets: 314001–315000#159|314159 Mattparker]]{{Efn|314159 are the first six digits of [[Pi]].}} was named in his honor. The citation highlights Parker's biennial "Pi Day challenges", stating that they have helped to popularize mathematics.<ref name="asteroid">{{cite journal |url=https://www.wgsbn-iau.org/files/Bulletins/V004/WGSBNBull_V004_011.pdf#page=20 |author=[[International Astronomical Union]] |title=New Names of Minor Planets |journal=WGSBN Bulletin |volume=4 |number=11 |date=15 August 2024 |access-date=15 August 2024 |quote=Matthew Thomas Parker (b. 1980) is an Australian recreational mathematician, author, and science communicator based in the United Kingdom. His “Stand-up Maths” YouTube channel has gained more than one million subscribers. Parker's Pi Day (March 14) challenges, where he calculates (by hand) π with the help of volunteers, have popularized mathematics.}}</ref><ref name="Piday2011">{{cite news |url=https://www.theguardian.com/science/blog/2011/mar/14/pi-day |title=Pi Day: Help yourself to a slice of infinite, transcendental pi |first=Matt |last=Parker |date=14 March 2011 |access-date=15 August 2024 |work=[[The Guardian]] |archive-url=https://web.archive.org/web/20231019161621/https://www.theguardian.com/science/blog/2011/mar/14/pi-day |archive-date=19 October 2023 |url-status=live}}</ref>

== Alternative dates ==
Pi Day is frequently observed on March 14 (3/14 in the ''month/day'' date format), but related celebrations have been held on alternative dates.

'''Pi Approximation Day''' is observed on [[July 22]] (22/7 in the ''day/month'' date format), since the [[Fraction (mathematics)|fraction]] {{frac|22|7}} is a common [[Approximations of π|approximation of {{pi}}]], which is accurate to two decimal places and dates from [[Archimedes]].<ref name="VerizonTodayInHistory">{{Cite web |title=Pi Approximation Day is celebrated today |url=http://www.thinkfinity.org/2010-07-22_pi-approximation-day |archive-url=https://web.archive.org/web/20101201010400/http://www.thinkfinity.org/2010-07-22_pi-approximation-day |archive-date=December 1, 2010 |access-date=January 30, 2011 |website=Today in History |publisher=Verizon Foundation}}</ref> In Indonesia, [[Date and time representation by country|a country that uses the DD/MM/YYYY date format]], some people celebrate Pi Day every July 22.<ref>{{Cite book |last=Purworini |first=D. |title=Pi Day celebration by local scientists in Indonesia |year=2016 |volume=7 |location=Anak Sudarti Foundation Bulletin |pages=7–8}}</ref>

'''[[Tau (mathematical constant)|Tau]] Day''', also known as '''Two-Pi Day''',<ref>{{Cite web |url=https://newsroom.lamresearch.com/Celebrating-Pi-Day-2017-and-More |title=Celebrating Pi Day 2017… and More! |first=Elizabeth |last=Pavel |date=March 13, 2017 |publisher=[[Lam Research]] |access-date=6 November 2024 |archive-url=https://web.archive.org/web/20230330164730/http://newsroom.lamresearch.com/Celebrating-Pi-Day-2017-and-More |archive-date=March 30, 2023 |url-status=live}}</ref> is observed on [[June 28]] (6/28 in the ''month/day'' format).<ref name="lightyears">{{Cite web |title=Tau Day: Why you should eat twice the pie |url=https://lightyears.blogs.cnn.com/2012/06/28/tau-day-why-you-should-eat-twice-the-pie/ |url-status=dead |archive-url=https://web.archive.org/web/20130112211136/http://lightyears.blogs.cnn.com/2012/06/28/tau-day-why-you-should-eat-twice-the-pie/ |archive-date=January 12, 2013}}</ref> The number [[Turn (angle)#Tau proposals|{{tau}}]], denoted by the Greek letter [[tau]], is the ratio of a circle's [[circumference]] to its [[radius]]; it equals 2{{pi}}, a common multiple in mathematical formulae, and approximately equals 6.28. Some have argued that {{tau}} is the clearer and more fundamental constant and that Tau Day should be celebrated alongside or instead of Pi Day.<ref>{{Cite news |date=March 13, 2018 |title=It's Pi Day today. But these people say we should refuse to celebrate it |language=en-GB |work=The Independent |url=https://www.independent.co.uk/news/science/pi-day-march-14-maths-google-doodle-pie-baking-celebrate-30-anniversary-a8254036.html |url-status=live |url-access=subscription |access-date=March 14, 2018 |archive-url=https://ghostarchive.org/archive/20220526/https://www.independent.co.uk/news/science/pi-day-march-14-maths-google-doodle-pie-baking-celebrate-30-anniversary-a8254036.html |archive-date=May 26, 2022}}</ref><ref>{{Cite news |date=March 14, 2013 |title=Pi Day Turns 25: Why We Celebrate an Irrational Number |url=https://news.nationalgeographic.com/news/2013/03/031314-pi-day-exploratorium-mathematics-pie-science/ |archive-url=https://web.archive.org/web/20130314185319/http://news.nationalgeographic.com/news/2013/03/031314-pi-day-exploratorium-mathematics-pie-science/ |url-status=dead |archive-date=March 14, 2013 |access-date=March 14, 2018}}</ref><ref>{{Cite web |last=BMJ |date=June 22, 2018 |title=Jeffrey Aronson: When I use a word ... The Days of Pi |url=https://blogs.bmj.com/bmj/2018/06/22/jeffrey-aronson-when-i-use-a-word-the-days-of-pi/ |access-date=May 2, 2023 |website=The BMJ |language=en-US}}</ref> Celebrants of this date jokingly suggest eating "twice the pie".<ref>{{Cite book |last=Bartholomew |first=Randyn Charles |url=https://www.scientificamerican.com/article/let-s-use-tau-it-s-easier-than-pi/ |title=Why Tau Trumps Pi |language=en}}</ref><ref>{{Cite news |last=Landau |first=Elizabeth |title=In case Pi Day wasn't enough, it's now 'Tau Day' on the Internet |language=en |work=CNN |url=http://www.cnn.com/2011/TECH/innovation/06/28/tau.day.math/index.html}}</ref><ref>{{Cite web |title=Tau Day – Come Eat Twice the (Pi)e |url=https://www.geocaching.com/geocache/GC56WHN_tau-day-come-eat-twice-the-pie?guid=19c50aa3-fe1c-4961-8df8-f03f3798de91 |language=en}}</ref>

Some also celebrate {{pi}} on [[November 10]], since it is the 314th day of the year (in leap years, on [[November 9]]).<ref>{{Cite web |title=Pi Day – Fun Holiday |url=https://www.timeanddate.com/holidays/fun/pi-day |access-date=March 13, 2022 |publisher=Timeanddate.com}}</ref>

==Gallery==
<gallery>
File:Pi pie2.jpg|Pi Pie at [[Delft University]]
File:Pi day sale.jpg|A grocery store selling pies for $3.14 on Pi Day
File:Cremepiday.png|Creme pie in celebration of Pi day showing the Greek letter and the first digits of Pi.
</gallery>

== See also ==
* [[Lists of holidays]]
* {{annotated link|National Mathematics Day (India)}}
* {{annotated link|Mole Day}}
* [[Sequential time]]
* [[Square Root Day]]
* {{annotated link|Doomsday rule}}
* [[Programmers' Day]]

==Notes==
{{Notelist}}

== References ==
{{reflist}}

==External links==
{{Commons category|Pi Day}}
{{Wiktionary|Pi Day}}
*[http://www.exploratorium.edu/pi/ Exploratorium's Pi Day Web Site]
*[https://www.idm314.org Official website of the International Day of Mathematics]
*[https://en.unesco.org/commemorations/mathematics UNESCO page on the International Day of Mathematics]
*[https://www.npr.org/sections/thetwo-way/2010/03/pi_day.html/ NPR provides a "Pi Rap" audiovideo]
*[http://www.idm314.org/ Pi Day]
*[http://www.math.utep.edu/Faculty/lesser/piday.html Professor Lesser's Pi Day page]
*[https://www.eeo.cn/webcast.php?courseKey=8a44be5970c129ba 数学漫谈] (A Tour  of Mathematics),  a public lecture (in Chinese) delivered by Professor [http://lsec.cc.ac.cn/~yyx/ Ya-xiang Yuan] (President of [[International Council for Industrial and Applied Mathematics]]) on 14 March 2020, the first International Day of Mathematics ([https://web.archive.org/web/20210706210819/ftp://lsec.cc.ac.cn/pub/home/yyx/2020.03.14.15.pdf slides])
{{Science commemorative events|state=collapsed}}
{{authority control}}

[[Category:1988 establishments in California]]
[[Category:March observances]]
[[Category:July observances]]
[[Category:Observances about science]]
[[Category:Pi]]
[[Category:Recurring events established in 1988]]
[[Category:Unofficial observances]]