Today is Pi Approximation Day. Pi Day and Pi Approximation Day are two unofficial holidays held to celebrate the mathematical constant pi. Pi Day is observed on March 14 (3/14 in American date format) due to pi being roughly equal to 3.14. Sometimes it is celebrated more specifically on March 14 at 1:59 p.m. (commonly known as the “Pi Minute”). If pi is rounded out to seven decimal places, it becomes 3.1415926, making March 14 at 1:59:26 p.m. the “Pi Second.” Pi Approximation Day may be observed on any of several dates, most often July 22 (22/7 in European date format is a popular approximation of pi). The first Pi Day celebration was held at San Francisco's Exploratorium in 1988, with staff and the public marching around one of its circular spaces and then consuming fruit pies. The Massachusetts Institute of Technology often mails out its acceptance letters to be delivered to prospective students on Pi Day.