PI DAY is a holiday held to celebrate the mathematical constant π (pi). Pi Day is observed on March 14th (3/14), due to π being roughly equal to 3.14. The Pi Minute is also sometimes celebrated on March 14 at 1:59 p.m. If π is truncated to seven decimal places, it becomes 3.1415926, making March 14 at 1:59:26 p.m., Pi Second (or sometimes March 14, 1592 at 6:53:58 a.m.).

The first Pi Day celebration was held at the San Francisco Exploratorium in 1988, with staff and public marching around one of its circular spaces, and then consuming fruit pies; the museum has since added pizza pies to its Pi Day menu.

The founder of Pi Day was Larry Shaw, a now retired physicist at the Exploratorium who still helps out with the celebrations. The Massachusetts Institute of Technology often mails out its acceptance letters to be delivered to prospective students on Pi Day.

Some also celebrate Pi Approximation Day in addition to Pi Day, which can fall on any of several dates:

  • April 26: The Earth has traveled two radians of its orbit by this day (April 25th in leap years); thus the entire orbit divided by the distance traveled equals pi
  • July 22: 22/7 in the more common day/month date format, an ancient approximation of pi
  • November 10: The 314th day of the year (November 9 in leap years)
  • December 21, 1:13 p.m.: The 355th day of the year (December 20 in leap years), celebrated at 1:13 for the Chinese approximation 355/113

On Pi Day, 2004, Daniel Tammet calculated and recited 22,514 decimal digits of pi.

Somewhat appropriately, it would seem, Albert Einstein was born on Pi Day, 1879.