Pi Day is observed on March 14 because the date, when written as 3/14, corresponds to the beginning of the decimal expansion of the most well-known mathematical constant, 3.14159. Pi is only a number by itself; there are countless others between 3 and 4.5 hours ago Although Pi Day is most usually celebrated on March 14, related events have also been conducted on other days. Since the fraction 227 is a common approximation of, which is accurate to two decimal places and dates from Archimedes, it is celebrated on July 22 (22/7 in the day/month format).