Pi Day is an annual celebration of the mathematical constant π (pi). Pi Day is observed on March 14 (3/14 in the month/day 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.

Pi has been calculated to over one trillion digits beyond its decimal point. As an irrational and transcendental number, it will continue infinitely without repetition or pattern. While only a handful of digits are needed for typical calculations, Pi’s infinite nature makes it a fun challenge to memorize, and to computationally calculate more and more digits. Physicist Larry Shaw started 14 March as Pi day at San Francisco’s Exploratorium. There he is known as the Prince of Pi. Since the exact value of pi can never be calculated, we can never find the accurate area or circumference of a circle.

Pi Approximation Day is observed on July 22 (22/7 in the day/month format), since the fraction ​22⁄7 is a common approximation of π, which is accurate to two decimal places and dates from Archimedes. Two Pi Day, also known as Tau Day, is lightly observed on June 28 (6/28 in the month/day format).

Many genius minds have a relation with the Pi day. Albert Einstein was born on Pi day. Stephen Hawkings died on 3/14, Pi day at the age of 76. 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, and the coincidental circular nature of a pie.