Tau Day is held to celebrate Tau (τ) on June 28th. Tau is a circle constant which is equal to 6.28318... (2π) or (2 * 3.14159...) and simplifies some equations.
A little background about Tau from Wikipedia:
In 1958 Albert Eagle proposed replacing π by τ = π/2 to simplify formula. However, no other authors are known to use tau in this way. Currently, some people use a different value for tau, τ = 6.283185... = 2π. arguing that τ is more natural than π, as the ratio of a circle's circumference to its radius rather than its diameter, and it simplifies many formulas. Celebrations of this number, because it approximately equals 6.28, by making 28 June "Tau Day" and eating "twice the pie", have been reported in the media. However this use of τ has not made its way into mainstream mathematics.
Read the Tau manifesto to learn about the origins of Tau Day by physicist Michael Hartl and the reasons Tau (6.28) is a better number to use as the circle constant instead of Pi (3.14) and the original article called why Pi is Wrong by mathematician Bob Palias from the University of Utah Department of Mathematics.