Pi Day and Pi Approximation Day are two unofficial holidays held to celebrate the mathematical constant π (pi). Pi Day is celebrated on March 14, or in the month/day date format as 3/14; since 3, 1 and 4 are the three most significant digits of π. Pi Approximation Day is held on July 22, or in the more common day/month date format as 22/7, which is an approximate value of π.
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Observation
Pi Day is observed on March 14, because of the Ancient Greek mathematician Archimedes' first rough approximation of π as being 3.14. (A few years later, Archimedes was able to calculate a much better approximation of π.) However, 22/7 is actually a closer approximation of π than 3.14 is. Thus, a more "accurate" Pi Day could be found in the more common calendar, 22/7, or July 22.
Sometimes the socalled Pi Minute is also commemorated. This one occurs twice on March 14 at 1:59 a.m., and 1:59 p.m. If π is truncated to seven decimal places, it becomes 3.1415926, making the Pi Second occur on March 14 at 1:59:26 a.m. (or 1:59:26 p.m.). If a 24hour clock is used, the Pi Second occurs just once yearly, on March 14 (3/14) at 1:59:26.
On March 14, 2016, the American way of writing the date (MM/DD/YY) will reflect five digits of pi (3/14/16) (π ≈ 3.1416) rather than three. The year before that, there will be a Pi Second accurate to 10 digits (3/14/15 9:26:54).
Celebration
There is a large variety of ways of celebrating Pi Day and most of them include eating pie and discussing the relevance of π.^{[1]} The first Pi Day celebration was held at the San Francisco Exploratorium in 1988, with staff and public marching around one of its circular spaces, then consuming fruit pies. The museum has since added pizza to its Pi Day menu.^{[2]} The founder of Pi Day was Larry Shaw,^{[3]} a nowretired physicist at the Exploratorium who still helps out with the celebrations.
The Massachusetts Institute of Technology often mails its acceptance (and rejection) letters to be delivered to prospective students on Pi Day.^{[4]}
Some also celebrate alternative Pi Days and/or Pi Approximation Days in addition to the two listed above;^{[citation needed]} these can fall on any of several dates:
 March 4: When 14% of the 3rd month has elapsed.
 April 5: When 3.14 months of the year have elapsed.
 April 26: The Earth has traveled two radians of its orbit on this day (April 25 in leap years), reckoning from the start of the calendar year on January 1. The distance travelled through the entire orbit around the sun, divided by the average distance to the sun, equals 2π; two radians equals ^{1}⁄_{π} of our orbit. This is celebrated exactly on the 41st second of the 23rd minute of the 4th hour on April 26 or the 116th day. (In leap years, it is celebrated exactly on the 3rd second of the 2nd minute of the 12th hour on April 25 or the 116th day.)
 November 10: The 314th day of the year (November 9 in leap years).
 December 21, 1:13 p.m.: The 355th day of the year (December 20 in leap years), celebrated at 1:13 for the Chinese approximation 355/113.
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