Pi Day and Pi Approximation Day are two unofficial holidays held to celebrate the Mathmetical Constant π (Pi). Pi Day is observed on March 14; Pi Approximation Day may be observed on any of several dates, most often July 22 (22/7 - in European date format - is a popular approximation of π). It is not clear how many people know about the holiday or celebrate it.
Pies for a celebration at the Massachusetts Institute of Technology.
March 14, written as 3-14 or 3/14 in the United States date format, represents the common three-digit approximation for the number π: 3.14. Pi Day is often celebrated at 1:59 p.m. in recognition of the six-digit approximation: 3.14159. Some, using a 24-hour clock, celebrate it at 1:59 a.m. or 3:09 p.m. (15:09) instead.
Pi Day is celebrated in a variety of ways. Parties or other observances may be held by mathematics, play Pinata, drink Pina Colada, eat Pizza, listen to the song "Pi" by Kate Bush, watch Pi, or recite Pi. The song 867-5309/Jennyis sometimes sung, replacing the digits with the first several digits of pi. The shape of the pie is sometimes square, due to the pronunciation of the equation for the surface area bounded by a circle = πr2, i.e., "pie are squared." departments in educational institutions. Harvard's Math department, for instance, has a pi recitation contest as well as a pi eating contest. Mathematics or science clubs might gather to consider the role that the number π has played in their lives and to imagine the world without π. During such an event, pi celebrants may approximate π, devise alternative values for π, eat Pie
It's unclear when Pi Day was first celebrated. The Exploratorium began an annual public celebration of Pi Day in 1987, with staff and public marching around one of its circular spaces, and then consuming fruit pies; the museum has since added pizza pies to its Pi Day menu.
Enthusiasts also note that the day happens to be Albert Einstein's birthday, among other famous birthdays on this day. Massachusetts Institute of Technology, known for its sometimes unconventional and quirky take on mathematics, often mails out its acceptance letters to be delivered to prospective students on Pi Day.
Some also enjoy celebrating "Pi Approximation Day" which can fall on any of several dates:
July 22 (22/7 in some date formats, a popular approximation of pi)
April 26 (when the distance of the Earth's orbit divided by the time it has traveled so far is equal to pi; on leap years the 25th)
November 10 (the 314th day of the year; on leap years the 9th)
December 21, 1:13 p.m. (the 355th day of the year, celebrated at 1:13 for the Chinese approximation 355/113)