Pi Day and Pi Approximation Day are two unofficial holidays held to celebrate the mathematical constant π (Pi). Pi Day is observed on March 14 (3/14 in American date format); Pi Approximation Day may be observed on any of several dates, most often July 22 (22/7 - in European date format - is a popular approximation of π).

The day happens to be Albert Einstein's birthday, among others, and it is common to sing "Happy birthday Dear Albert". Massachusetts Institute of Technology often mails out its acceptance letters to be delivered to prospective students on Pi Day.[1]