Pi Day

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Pi Pie at Delft University

Pies for a celebration at the Massachusetts Institute of Technology

Free pie being prepared at the University of Waterloo

Larry Shaw, the founder of Pi Day at the Exploratorium

Pi Day and Pi Approximation Day are two holidays held to celebrate the mathematical constant π (pi). Pi Day is observed on March 14 (3/14 in American date format), due to π being equal to roughly 3.14. Sometimes it is celebrated on March 14 at 1:59 p.m. If π is truncated to seven decimal places, it becomes 3.1415926, making March 14 at 1:59:26 p.m., Pi Second (or sometimes March 14, 1592 at 6:53:58 a.m.). Pi Approximation Day may be observed on any of several dates, most often July 22 (22/7 (European date format) is a popular approximation of π). March 14 also happens to be Albert Einstein's birthday.

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, and then consuming fruit pies; the museum has since added pizza pies to its Pi Day menu.^[1] The founder of Pi Day, the "Prince of Pi", is Larry Shaw,^[2] now retired from the Exploratorium, but still helping out with the celebrations. They have also recently added the first Pi Day celebrations in Second Life.

The Massachusetts Institute of Technology often mails out its acceptance letters to be delivered to prospective students on Pi Day.^[3]

Some also celebrate Pi Approximation Day in addition to Pi Day, which can fall on any of several dates:

July 22: 22/7 in nearly all date formats, an ancient approximation of pi
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

On March 14, 2004, Daniel Tammet recited from memory 22514 decimal digits of pi.^[4]