Pi, Greek letter (Π), is of course the symbol for the ratio of the circumference of a circle to its diameter. Pi = 3.1415926535... (This is pi to just ten decimal places; it is an irrational number, meaning the digits continue infinitely without repeating.) Math enthusiasts around the world celebrate Pi Day on March 14th, i.e. 3/14 in the American dating convention, since the first three digits of PI are 3.14 (March 14 also happens to be Albert Einstein's birthday).  "Pi Moment" occurs on that day just before 3:10 pm (more precisely, 15:9:26.5).

The San Francisco Exploratorium celebrated the first Pi Day in 1988, with staff and public marching around one of its circular spaces, and then consuming fruit pies. Since then, the museum has added pizza pies to its Pi Day menu. The founder of Pi Day, the "Prince of Pi," is Larry Shaw, now retired from the Exploratorium, but still helping with the celebrations. The Massachusetts Institute of Technology often mails out its acceptance letters to be delivered to prospective students on Pi Day.

Celebrate Pi Day by doing anything to get students interested in mathematics. Teachers can organize many unique activities for students to generate interest. It is also used to inform students as well as the rest of the public about Pi, its history and applications. Pi Day parties in math and science classes are common, utilizing different types of pies, all the while teaching students about Pi.