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Pi Approximation Day is held on July 22 in the day/month date format because it is the approximate value of π (22/7 = 3.14).
What is Pi (π)?
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The lower case Greek letter π is used because it is the first letter of the Greek work περίμετρος (perimeter), which probably refers to its use in the formula perimeter divided by diameter equals the constant for all circles. The concept of π has become the most common ground between mathematicians and non-mathematicians.
What do people do?
There are many activities that celebrate Pi Day such as games, creating some type of pi ambiance, eating “pi” foods, converting things into pi, making strange mathematical endeavors like having a contest to see who knows the most digits of pi. Many people celebrate Pi Day by eating pie and discussing the relevance of π. Many teachers will use this date to engage students in activities related to pi by singing songs and carols about pi and developing pi projects.
Mathematicians, teachers, museum directors, math students of all ages and other enthusiasts celebrate the number with pi recitations, pie-baking, pie-eating contests and math-related activities.
The First Pi Day
Pi Day celebrations was founded by Larry Shaw and it was first held at the San Francisco Exploratorium in 1988. The celebrations began with the public and museum staff marching around a circular space and then eating fruit pies. The museum has since then added pizza to its menu and has grown to include activities such as creating Pi puns, Pi-related antics, and many other activities that involve Pi.
Alternative Pi Days and Pi Approximation Days
Pi Day and/or Pi Approximation Day can be celebrated on other calendar dates such as:
July 22: When 22 is divided by 7, it equals 3.14.
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). 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.
Source: timeanddate.com (sligtly abridged)
You can read more about Pi Day on Teaching & Learning @ Pi Day 2012.
If you are looking for teachers' resources, check TeachPi, where you'll find ideas for Pi Day activities, learning and entertainment!