Tau Day is an unofficial holiday that operates on a very simple premise: that PI is wrong. This holiday celebrates Tau—a replacement for PI that sets the circle constant at approximately 6.28. It is considered to be antiPi because Pi equals a circle’s circumference divided by its diameter (3.14), while Tau represents a circle’s circumference divided by its radius. Tau is considered by some to be a lot more accurate way to find a circle’s circumference, but not every in the mathematics community agrees. As a result, this holiday is still somewhat on the fringes of mathematical holidays. Regardless of whether this holiday is used as a bookend with PI Day (celebrated on March 14th) or whether it’s celebrated as a holiday in its own right, this day is sure to get those people excited about math ready to get the party on.
The History Of Tau Day
Before we can discuss the history of Tau Day, we first have to discuss the history of Tau. Tau was first approximated during the third century by Archimedes. Archimedes used PI to calculate the circumferences of circles, and it had remained so for eighteen hundred years. However, that changed in 2010 when Michael Hartl released The Tau Manifesto – a detailed document that showed why the constant of Pi is wrong and why the constant of Tau should be used instead. Since that time it has been celebrated, but not always accepted all over the world. Michael Hartl was an educator, entrepreneur, and author when he created this holiday and he had previously taught computation and theoretical physics at Caltech (California Institute of Technology). He also knows PI to 50 decimal places and Tau to 52 decimal places. For everyone interested, Tau is 6.283185. If you thought that we were going to type Tau to 52 decimal places, then we’re afraid that we’re going to have to disappoint you.
Celebrating Tau Day
People celebrate this holiday in a number of different ways. For instance, since the value of Tau is twice as much as that of Pi, many people will eat double the amount of pie on this day. Another way to celebrate the day is by wearing a Tau Tshirt or reading The Tau Manifesto. Perhaps an equally acceptable way is to buy a pie and then destroy it in much the same way as Tau destroyed PI (although, we have to admit that we think that’s a terrible waste of pie). If you’re a math teacher, you can use this holiday to involve your students in mathrelated activities that are not only fun but also beneficial for their education. While you’re online, don’t forget to use the hashtag #TauDay on your social media accounts. Whether you’re looking for a holiday to replace Pi Day or are simply looking for another mathematically themed holiday, Tau Day is a day that’s just waiting for you to celebrate it. So plan your favorite activity and enjoy this holiday to all of its decimal points.
When is Tau Day? 

This year (2023)  June 28 (Wednesday) 
Next year (2024)  June 28 (Friday) 
Last year (2022)  June 28 (Tuesday) 