Happy τ day! τ≈6.2831853, of course, is the fundamental circle constant which represents the ratio of any circle’s circumference to its radius. (In the past people have also used the symbol “π” to represent half of τ; perhaps you’ve heard of it.)
To learn more about the glories of τ, I recommend reading the Tau Manifesto by Michael Hartl.
We celebrate τ day by eating twice as much pie as we really ought to.