If it’s true that on St. Patrick’s Day everyone can be Irish, then just three days earlier, on March 14, everyone can be a mathematician. For this is Pi Day, a celebration of that famous irrational number and all things to do with circles. In 2013, Pi Day was held for the twenty-fifth time, and although it may not yet be as well known as other landmark days on the calendar, that could be about to change.

A short history of Pi Day

Pi Day began at San Francisco’s Exploratorium in 1988, when a physicist named Larry Shaw had a ‘Eureka!’ moment. He noticed that March 14 corresponded with the first numbers of pi – 3.14 – and proclaimed “It’s Pi Day. Let’s celebrate.” Shaw organized a parade of his fellow workers around one of the venue’s circular spaces, after which they all ate fruit pies. Yes, it all sounds a little crazy, but what better way to recognize an irrational number?

The fact that it was also Einstein’s birthday – he was born on this day in 1879 – apparently had nothing to do with it. That is just one of the strange and wonderful coincidences associated with Pi Day. Want another? Write 3.14 on a piece of paper and hold it up to a mirror to see what it spells!

Pi is ‘irrational’, by the way, because its decimals continue infinitely, without any repeating pattern, and it can’t be expressed as a precise ratio of two integers. The closest approximation, which most people remember from their high school math classes, is 22/7. The number, a mathematical constant which expresses the relationship between a circle’s circumference and its diameter, has been known since Babylonian times at least, though it wasn’t actually named ‘pi’ until the mid-18th century.

From its humble beginnings in 1988, Pi Day began attracting attention from other scientists and mathematicians, and was soon being celebrated in schools and colleges. In 2009, the U.S. House of Representatives passed a non-binding resolution to officially recognize March 14 as National Pi Day. Because one day of the year is really not enough to celebrate such a fascinating mathematical principle, Pi Approximation Day – on July 22 – was recently added to the calendar and, since 2010, Tau Day has been commemorated on June 28 by those who believe that twice pi – 6.28, or the ratio between a circle’s circumference and its radius – is a much more important number.

How to celebrate Pi Day

Pi Day is recognised in many creative ways, from eating discounted slices of pizza to writing ‘piems’; poems in which each word corresponds to a digit of pi. MIT continued its tradition of posting acceptance letters on March 14, and this year they also acknowledged tau-supporters by informing high school seniors online at precisely 6.28 pm. Meanwhile, at the Exploratorium, the big day’s origins were celebrated with a wide range of pi-related activities which have been proudly captured on Facebook.

It’s also a great day for teachers and students to think about the universal language of mathematics in a fresh and entertaining way. Many creative suggestions are available at piday.org, and for the more musically minded, there’s a very clever rap - based on Eminem’s “Lose Yourself” - which can be downloaded here.

In 2013, millions of people around the world will discuss the merits of pi, march in circles, and gorge themselves on pies and pizzas. A remarkable few will even recite pi to thousands of decimal places, though it is unlikely that any will get close to the current record held by Chinese student Chao Lu, who in 2005 recited pi from memory to 67,890 places.

If you missed Pi Day this year, don’t worry. What goes around comes around, and in the years to come, one senses that March 14 will increasingly be recognized as a day that’s definitely not for squares!