Pi
People generally celebrate March 14 as Pi Day, because March 14 is also written as 3/14, which represents the first three numbers 3.14 in the approximation of pi. Pi day was started in 1988 by physicist Larry Shaw at the San Francisco Exploratorium. On March 11, 2009, almost 21 years later  U.S. House Of Representatives passed a resolution proclaiming March 14 to be celebrated as National Pi Day every year.