( CNN ) Pi Day is going to be extra special this year , as 2015 stretches the symbolic March 14 celebration out a little longer to 3.1415 . And if you mark pi at 9:26.53 in the morning or night , you 're just a little more in line with the celebrated irrational number that never ends . To 31 decimal places , pi is 3.1415926535897932384626433832795 . A few more tidbits about pi and Pi Day : . About pi . Pi is the ratio of the circumference of a circle to its diameter . It 's not equal to the ratio of any two whole numbers , so an approximation -- 22/7 -- is used in many calculations . Pi is essential in architecture and construction and was used frequently by early astronomers . Pi has been known for about 4,000 years , but it started to be called by the Greek letter only in the 1700s . The origin of Pi Day . Pi Day started 27 years ago at San Francisco 's Exploratorium . Physicist Larry Shaw , who worked in the electronics group at the museum , started celebrating pi on March 14 , 1988 , primarily with museum staffers . The tradition has grown to embrace math enthusiasts from all walks of life . Celebrations . This year , the Exploratorium is hosting a day-long celebration at its facility on Pier 15 , including a Pi Procession , servings of pie and a pizza pie dough-tossing event . Pi Day is also Albert Einstein 's birthday . In Princeton , New Jersey , where Einstein lived for more than two decades , signs of the scientist permeate the Pi Day festivities , from birthday parties at the Historical Society of Princeton to an Einstein lookalike contest . Plus the requisite pie-eating , pie-throwing and pizza pie creation . For more about pi , visit www.piday.org . CNN 's Elizabeth Landau contributed to this report .
Pi Day is celebrated March 14 in honor of the famed irrational number . The Pi Day Foundation was founded by a group of students at MIT and Stanford University to celebrate the discovery that pi equals 3.141592653589793238469...