March 14th. It's American Pi Day because of the way dates are written in in the American style, with the month preceding the day. In countries that write the day preceding the month, Pi Day is more properly celebrated on 22nd of July.
Albert Einstein was born on March 14th. He had to move to America in the 1930s in order to take advantage of it being American Pi Day there.
by DrEnglish March 14, 2010