Pi Day is an annual celebration of the mathematical constant π (pi) in the US.

• Pi Day is observed on March 14 (3/14 in the month/day date format) since 3, 1, and 4 are the first three significant digits of π.
• In 2009, the United States House of Representatives supported the designation of Pi Day.
• Archimedes of Syracuse (about 287–212 B.C.) is credited with doing the first calculation of Pi, while British mathematician William Jones came up with the Greek letter and symbol for the figure in 1706 — which was later popularized by Swiss mathematician Leonhard Euler, beginning in 1737.
• Last year’s date — 3.14.2015 — was especially significant because it matched the first four digits after the decimal point.
• This year, some math lovers have already started calling 3.14.16 “Rounded Pi Day,” rounding up those four digits.
• Pi Approximation Day is observed on July 22 (22/7 in the day/month date format), since the fraction 22⁄7 is a common approximation of π, which is accurate to two decimal places and dates from Archimedes.
• Pi Day has been observed in many ways, including eating pie, throwing pies and discussing the significance of the number π, due to a pun based on the words “pi” and “pie” being homophones in English (pronunciation: /paɪ/), as well as pies tending to be round, and thus related to π.