Happy 3.14159… Day!! Pi Day is celebrated on March 14th (3/14) each year and is a celebration of the mathematical constant Pi (π), which is the ratio of a circle's circumference to its diameter. Pi Day was first celebrated in 1988 by Larry Shaw, a physicist at the Exploratorium in San Francisco, California. Shaw organized a celebration that included a circular parade and the consumption of pies, both sweet and savory.Pi Day gained more widespread recognition in the 2000s, with various organizati