FLASH WARNING Happy PI.E/3.14 day! In case you don't know, pi (π) based of the lowercase Greek letter, is the quotient of ANY circle's circumference to its diameter. Its an Irrational number, meaning it has random endless limitless digits that never end. The reason why pi day is on march 14 is because the date is 3/14. just like 3.14. (the approximate amount for pi!) 3/14 Circumference/Diameter How many digits of pi have you memorized? So far, I've memorized about 200.