Times may change on a dime in our tumultuous world, but one thing always remains the same: pi, the irrational-mathematical constant, the ol’ circumference/diameter split, the big 3.14159265358979323…. you get the idea.

As immutable as pi is, so is Pi (π) Day, a celebration that San Francisco’s Exploratorium invented back in 1988 that has become an international sensation. Taking place on March 14 every year, Pi Day in the Bay typically involves math-themed activities and presentations, a pi parade with a brass band that begins at 1:59 p.m. (the numbers that follow 3.14) and then literal pie – free slices of pizza for everyone who’s paid museum admission.

In 2025, the Exploratoriuim is promising a “celestial twist” to the festivities, as the holiday occurs on the same day as a total lunar eclipse – the spookily named “Blood Moon,” which will appear as dark red in the sky. What’s being planned? Well, so far there are demonstrations of how female engineers and astronauts use pi in their space-exploration work. Aside from pizza, there will also be Moon Pies to scarf up. And that’s probably enough pie for one day for people watching their own circumferences.

The Exploratorium in San Francisco will hold its annual Pi (π) Day festival on March 14, 2025, with a parade and free pizza for attendees. (Gayle Laird/Exploratorium) 

Details: Event takes place noon-5 p.m. March 14 at Pier 15 (Embarcadero at Green St,), San Francisco; $40 daytime tickets, exploratorium.edu/pi