Pi Day A group of middle school students forming the Greek letter π in honor of Pi Day. The holiday, a time to celebrate the number pi (which is about 3.14) and math in general, occurs each year on March 14 (or 3/14). (more) What Is Pi Day? Written by Erik Gregersen Erik Gregersen is a senior editor at Encyclopaedia Britannica, specializing in the physical sciences and technology. Before joining Britannica in 2007, he worked at the University of Chicago Press on the... Erik Gregersen Fact-checked by Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Britannica Editors Last updated Mar. 12, 2026 •History What Is Pi Day?, Pi Day is a celebration of the never-ending mathematical constant pi (π), which relates the circumference C of a circle to its diameter d; that is, π = C/d.The numerical value of pi to five decimal places is 3.14159; hence, Pi Day is celebrated on March 14 (3/14). Pi Day is often celebrated with pies or pizzas decorated with the Greek letter π. American physicist Larry Shaw, who worked at the...

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