In case you missed it, March 14th was an important international holiday. Every year, math enthusiasts worldwide celebrate the date as Pi Day. March 14th. 3/14. 3.14. Pi. Get it? If you’d like a higher degree of accuracy, you can celebrate Pi Minute at 1:59 on that date (as in 3.14159). Or why not Pi Second at 26 seconds into the Pi Minute (3.1415926)?

“It’s crazy! It’s irrational!” crows the website of the Exploratorium, San Francisco’s famously quirky hands-on science museum. The Exploratorium invented the holiday twenty-one years ago. In a delightful coincidence, Pi Day coincides with Albert Einstein’s birthday. Exploratorium revelers circumambulate the “Pi Shrine” 3.14 times while singing Happy Birthday to Albert.

Pi Day celebrations have spread to schools. Just over a year ago, I visited Singapore American School to give a week’s worth of presentations and I found parent volunteers serving pie to appreciative students whose math teachers were trying to sweeten their understanding of the world’s most famous irrational number. Just as pi is endless, so is the list of activities, from memory challenges and problem solving to finding how pi is connected to hat size … and writing a new form of poetry called “pi-ku,” which uses a 3-1-4 syllable pattern instead of haiku’s 5-7-5.

It is indeed the mysteriousness of pi that makes it so fascinating. For 3,500 years, according to David Blatner, author of The Joy of Pi, pi-lovers have tried to solve the “puzzle of pi” — calculating the exact ratio of a circle’s circumference to its diameter. But there is no such thing as “exact.” No matter how successful, pi can only be estimated.

A refresher course for the pi-challenged: The 16th letter of the Greek alphabet, ? or “pi,” is used to represent the number you get when you divide a circle’s circumference (the distance around) by its diameter (distance across, through the center). Try it on any circle with a ruler and string and you’ll get something a little over 3 1/8 or approximately 22/7 (some have therefore proposed the 22nd of July for Pi Day). Measured with a little more precision, the ratio comes out to 3.14. But don’t stop there. Pi is an irrational number, meaning that, expressed as a decimal, its digits go on forever without a repeating pattern. Hence the obsession of some with memorizing pi to 100, even 1,000 places. As a Pi Day gift from 5th graders at a school I visited this year on March 15th, I received a sheet of paper with pi written out to 10,000 digits. In 2002, a computer scientist found 1.24 trillion digits. Never mind that astrophysicists calculating the size of galaxies don’t seem to need an accuracy of pi any greater than 10 to 15 digits. Playing with pi offers endless hours of good, clean mathematical fun. So what if it’s irrational.

Happy (belated) Pi Day, everybody!