Founded in 1988 at the Exploratorium, Pi (π) Day has become an international holiday, celebrated live and online all around the world. The numbers in the date (3/14) match the first three digits of the mathematical constant pi (π).

What is π anyway? Divide any circle’s circumference by its diameter; the answer (whether for a pie plate or a planet) is always approximately 3.14, a number we represent with the Greek letter π. Keep calculating π’s digits with more and more accuracy—as mathematicians have been doing for 4,000 years—and you’ll discover they go on literally forever, with no pattern.

Celebrate the endless number π with mathematical antics and plenty of pie.

Learn about how π inspired a celebration and became a national holiday.

Use a touchscreen to search for any number hidden in the first 3 million digits of pi.

This circular piece of Exploratorium history has held pride of place in π Day festivities since 1988.