March 14 is Pi* Day, celebrated by math geeks. But Michael Hartl says we should celebrate ‘Tau Day’ instead, in his Tau manifesto. Tau is an alternative circle constant referred to by the Greek letter τ that equals 2π, or approximately 6.28. (So Tau Day would be June 28).
*Pi is the Greek letter used in math for the ratio of a circle’s circumference to its diameter. No matter the size of the circle, this is always the same value (approx. 3.14): a mathematical constant.
One big thing that Tau fixes, is radian angles (see diagram). It also makes sine and cosine functions simpler, and higher math like integrals in polar coordinates, the Fourier transform, and Cauchy’s integral, simpler.
Postscript: I found 
this great cartoon tribute to theoretical physicist Stephen Hawking that had passed away on Mar 14.
He once said ‘The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge’.