Pi Day and Pi Approximation Day are two unofficial holidays held to celebrate the mathematical constant ð (Pi). Pi Day is observed on March 14; 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 ð).

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.[citation needed]

Some also enjoy celebrating "Pi Approximation Day" which can fall on any of several dates:

http://www.piday.org/
March 14 the approximation for Pi: 3.14
April 26 (when the distance of the Earth's orbit divided by the time it has traveled so far is equal to pi; on leap years the 25th)[citation needed]
July 22 (22/7 in the majority of date formats, a popular approximation of pi)
November 10 (the 314th day of the year; on leap years the 9th)
December 21, 1:13 p.m. (the 355th day of the year, celebrated at 1:13 for the Chinese approximation 355/113