What is Pi? Pi (π) is approximately 3.14159, the mysterious mathematical constant which represents the ratio of any circle's circumference to its diameter. I say "approximately" because Pi itself is what is termed an "irrational number". That means it never repeats or ends. It keeps going and going forever.
Pi Day is celebrated on March 14 (3/14) each year in honour of Pi. It was first founded in 1988 at the Exploratorium in San Francisco by physicist Larry Shaw, and in 2009 the US House of Representatives designated March 13 as "National Pi Day". Schools around the nation take the occasion of the holiday to explore the significance of this number. It is currently treated as a day of mathematics, fun, and an excuse to have more pie.