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 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 for the mathematical constant Tau, is observed on June 28 (6/28 in the month/day format).

In 1988, the earliest known official or large-scale celebration of Pi Day was organized by Larry Shaw at the San Francisco Exploratorium, where Shaw worked as a physicist,[8] with staff and public marching around one of its circular spaces, then consuming fruit pies.[9] The Exploratorium continues to hold Pi Day celebrations.

On March 12, 2009, the U.S. House of Representatives passed a non-binding resolution (111 H. Res. 224), recognizing March 14, 2009 as National Pi Day. For Pi Day 2010, Google presented a Google Doodle celebrating the holiday, with the word Google laid over images of circles and pi symbols; and for the 30th anniversary in 2018, it was a Dominique Ansel pie with the circumference divided by its diameter.

The entire month of March 2014 (3/14) was observed by some as “Pi Month”. In the year 2015, March 14 was celebrated as “Super Pi Day”. It had special significance, as the date is written as 3/14/15 in month/day/year format. At 9:26:53, the date and time together represented the first 10 digits of π.