Tau Day
June 28 · Level 3 · #1670 of 2238
Tau Day is for maths fans who think the circle's key constant should be tau, equal to 2π or about 6.28, rather than pi. That is why it lands on 28 June, or 6/28.
How it is celebrated: enthusiasts argue over pi versus tau, draw circles and play with radians, and sit down to pies at the table, ideally two of them, since tau is two pi.
Michael Hartl published his Tau Manifesto on 28 June 2010. In it he argued that a full turn is more naturally written as a single tau, and a quarter turn as τ/4.