Pi Day and Pi Approximation Day are two holidays held to celebrate the mathematical constant π (pi) (in the mm/dd date notation: 3/14); since 3, 1 and 4 are the first three digits of π. March 14 is also the birthday of Albert Einstein and the two events are sometimes celebrated together.
Pi Approximation Day is observed on July 22, because of Archimedes' popular approximation of π being 22/7. However, this may be considered misleading, as all cited dates are "approximation days" (as π is an irrational number) and 22/7 is actually a closer approximation of π than 3.14 is. Typically, March 14 is more popular for countries using the month/day format (22/7 being an impossible date in this format), and the 22nd of July is more popular for countries using the day/month format (since 3/14 and 31/4 are impossible dates in this format).
Sometimes Pi Minute is also celebrated; this occurs 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.).
There are a large variety of ways of celebrating Pi Day and most of them include eating pie and discussing the relevance of π. 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, then consuming fruit pies. The museum has since added pizza to its Pi Day menu. The founder of Pi Day was Larry Shaw, a now-retired physicist at the Exploratorium who still helps out with the celebrations.
The Massachusetts Institute of Technology often mails its acceptance letters to be delivered to prospective students on Pi Day.