(CNN) - Thursday is the 14th day of the third month...3, 1, and 4 or 3.14.
Those digits are crucial because they make March 14th National Pi Day, not as in apple or pizza pie but pie as in the mathematical constant 3.14.
It's represented by the Greek letter Pi.
Pi is the ratio of a circle's circumference to its diameter.
According to nationaldaycalendar.com, the first major observance of Pi Day was in 1988 at the San Franciso exploratorium.