Tau Day, an anti-Pi Day holiday, is an unofficial observance held every year since 2010 on June 28, or 06/28 as written in the American date format.
The day began as a way to recognize tau, τ, a ratio constant describing the relationship between the circumference and the radius of a circle. In recent years, some mathematicians and physicists have argued for the replacement of pi (π) by tau.
The concept of replacing pi by tau as a better way to describe the relationship between a circle’s circumference and its radius was first brought up by mathematics professor, Bob Palais in the Mathematical Intelligencer in an article called π is wrong!. This was followed by the publishing of the Tau Manifesto by physicist Michael Hartl. Both of these articles called for the replacement of pi by tau. Tau, the ratio of a circle's circumference to its radius, is equal to 2 π or 6.283.
Not Widely Accepted
Tau is not yet widely accepted within the scientific community, though there is a large community of scientists and technologists who have called for the replacement of pi by tau.