Pi Day celebrates the mathematical constant π (3.14). It is celebrated in countries that follow around the world because the digits in the date, March 14 or 3/14, are the first three digits of π (3.14). Whilst we don’t write our date in the same way, it’s a good day to recognise the mathematical constant and all things mathematics, particularly geometry. 

Pi Day was founded by Physicist Larry Shaw in 1988.