Pi Day is a happy day, no matter how you slice it.
The Babylonians first approximated pi in their construction efforts, so pi has a long and worthy history. The Babylonions began by assigning pi = 3, then later adapted that to pi = 3.125.
The Egyptians, in the Rhind Papyrus, gave pi a value of 3.160.
Pi has a lengthy history.
Congress officially designated March 14 as Pi Day in 2009. Jonathan Zuck (president of Association of Competitive Technology) probably had a hand in this decision by quietly delivering pies to colleagues and congressional staffers for several years before the proclamation. I don't know if he still delivers pies to them or not. I wonder if the congrescritters who voted for it know that it's the mathematical number they were promoting through this proclamation and not fruity pies?
A lot of people celebrate Pi Day with pie, because - pie.
Because pi is such a popular number, there are a plethora of pi approximation days, as well:
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
Places to read about Pi Day: