When is pi (mathematical constant π) day celebrated?

Pi is the ratio of the circumference of a circle to its diameter and is represented by the mathematical constant π. It is celebrated on March 14 (3/14 in the month/day date format) since 3, 1, and 4 are the first three significant digits of π. It's rounded up to 3.14159 because the actual ratio has been calculated to over one trillion digits beyond the decimal point. The number π has been represented by the Greek letter "π" since the mid-18th century, though it is also sometimes spelled out as "pi".

The earliest known official or large-scale celebration of Pi Day was organized by physicist Larry Shaw in 1988 at the San Francisco Exploratorium, where staff and public marched around one of its circular spaces, then consumed fruit pies. The Exploratorium continues to hold Pi Day celebrations. In 2009, the United States House of Representatives supported the designation of Pi Day.