Forget about Pi Day, today we celebrate something twice as good: Tau Day. For far too long, Pi has been the bride and Tau has been the bridesmaid. As Michael Hartl points out in The Tau Manifesto, "Pi is a confusing and unnatural choice for the circle constant." He is giving a talk at the California Institute of Technology based on the Manifesto, with pie served at the end. "Twice as many as you might expect," he says.