March 14 is celebrated as Pi Day to commemorate the mathematical constant π
Photo source: Green Growth Cascadia
In 2009 US House of Representatives supported the designation of Pi Day, thus approving March 14 as the National Pi Day. Pi Day is celebrated on March 14 (3/14 in month/day date format), since 3, 1 and 4 are the three most significant digits of π in the decimal form.
Pi day is celebrated to promote significance of math and engage people in discussions about π and math. Some people celebrate by eating a pie since pi and pie are homophones.