Tau Day

Tau Day is an unofficial holiday which is celebrated on June 28th annually. The purpose of this holiday is to celebrate Tau – a replacement for PI that sets the circle constant at approximately 6.28. It is considered to be anti-Pi because Pi equals a circle’s circumference divided by its diameter (3.14), while Tau represents a circle’s circumference divided by its radius. While some scientists have argued that Tau should replace Pi, Tau is still not widely accepted throughout the scientific community.

History of Tau Day

Tau Day was launched 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 all over the world.

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 T-shirt or reading The Tau Manifesto.

If you’re a math teacher, you can use this holiday to involve your students in math-related activities that are not only fun but also beneficial for their education.


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.

Where is Tau Day celebrated?

There is no specific location where this holiday is celebrated.
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