Pi Day is an unofficial holiday commemorating the mathematical constant π (pi). Pi Day is celebrated on March 14 (or 3/14 in month/day date format), since 3, 1 and 4 are the three most significant digits of π in the decimal form. In 2009, the United States House of Representatives supported the designation of Pi Day.^[2]

Pi Approximation Day is held on July 22 (or 22/7 in day/month date format), since the fraction ²²⁄₇ is a common approximation of π.^[3]