Pi Day and Pi Approximation Day are two holidays held to celebrate the mathematical constant π (pi). Pi Day is observed on March 14 (3/14 in American date format), due to π being roughly equal to 3.14. Pi Approximation Day is observed on July 22, due to π being roughly equal to 22/7.

Pi Day is also sometimes celebrated on March 14 at 1:59 p.m. If π is truncated to seven decimal places, it becomes 3.1415926, making March 14 at 1:59:26 p.m., Pi Second (or sometimes March 14, 1592 at 6:53:58 a.m.). March 14 also happens to be Albert Einstein's birthday.

(... from Wikipedia on 2009-01-01 08:40:10 )