Pi Day is the brainchild of American physicist Larry Shaw, who created the holiday at San Francisco Exploratorium in 1987. It was Sara Shaw, Larry's then 14-year-old daughter, who accidentally found that Albert Einstein's birthday (March 14, 1879) overlapped the Pi Day in 1989 at the 3rd Pi Day. This founding in turn gradually promotes this special date to reach an international fame. Link
From Wikipedia:
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 equal to roughly 3.14. Sometimes it is celebrated on March 14 at 1:59 p.m. (commonly known as Pi Minute). 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.). Pi Approximation Day may be observed on any of several dates, most often July 22 (22/7 (European date format) is a popular approximation of π).
Some trivia about Pi Day and Pi Approximation Day
MIT often mails out its acceptance letters to be delivered to prospective students on Pi Day
July 22: 22/7 in nearly all date formats, an ancient approximation of pi
November 10: The 314th day of the year (November 9 in leap years)
December 21, 1:13 p.m.: The 355th day of the year (December 20 in leap years), celebrated at 1:13 for the Chinese approximation 355/113
On March 14, 2004, Daniel Tammet recited from memory 22514 decimal digits of pi.
Daniel "Pie Man " Tammet at Late Show with David Letterman (27/04/2005)In 2006, Japanese man Akira Haraguchi recited Pi to 100,000 Digits