What Is Pi Day? Why It Falls on March 14

A date that’s also a number

On March 14, in thousands of schools and science museums around the world, people celebrate Pi Day. The name rests on a calendar coincidence: in the United States, dates are written month first, so March 14 becomes 3/14 — the same as pi’s first digits, the constant behind every circle, covered in the Recap on what pi is.

The idea isn’t old. According to Wikipedia, it started in 1988 during a staff retreat at San Francisco’s Exploratorium, a hands-on science museum. Larry Shaw, a physicist at the museum, was talking with a few colleagues, including Ron Hipschman, about mathematical constants, and noticed the link between the date and pi’s digits.

Real-world example: in a country where dates are written day/month, the pun doesn’t carry over the same way: 14/3 doesn’t read as 3.14. That’s part of why Pi Day stays tied to the US date format, even where it’s celebrated outside the United States.

From a staff party to a public parade

The first edition, in 1988, was small: just Exploratorium staff, a march around one of the museum’s circular spaces, and fruit pies afterward. The following year the celebration opened to all visitors, and it has repeated every year since, even through the museum’s moves to new buildings.

The public version settled into a set routine: a parade at 1:59 p.m. (the digits that follow 3.14 in pi), with participants carrying signs showing one digit each, a gathering point called the “pi shrine,” fruit pie and pizza, and a round of “Happy Birthday” sung to Albert Einstein, which has become part of the Exploratorium’s yearly tradition.

Larry Shaw never took credit for it. He liked to say he was just the one holding the pole.

Recognition from the US Congress

On March 12, 2009, the US House of Representatives passed resolution H.Res. 224, recognizing March 14 as “National Pi Day.” According to the resolution’s text, the date is tied to pi’s approximation as 3.14; the document formally supports celebrating the day beyond US borders and encourages schools and teachers to build activities around the number it’s named for. It’s a non-binding resolution, though: it doesn’t close a single school or office, and it doesn’t require any state or teacher to observe it.

A holiday you can eat

The most common thread running through Pi Day is pie: “pie” sounds like the first syllable of “pi,” and that pun has shaped how the day gets marked everywhere. Eating, throwing, and giving away pie has become almost as much a part of the ritual as talking about the number behind the name.

Anyone curious about the number itself, rather than the holiday, can find out why pi never ends in the Recap on irrational numbers: pi is one of those numbers that can’t be written as an exact fraction, which is why its decimal digits keep going without ever repeating.