Pi is a one of the most commonly known irrational numbers. It represents the ratio between the circumference of a circle (the distance around) to its diameter (the distance across). Because Pi is an irrational number, it is impossible to calculate exactly, but in any circle the circumference is approximately 3.14 times its diameter.
Pi Day is celebrated annually on March 14 and especifically at 1:59. This is to commenorate the first few digits of pi which 3.14159.