Tau Day is a celebration of the circle constant $\tau = C/r =$ $ 6.283185\ldots$ Founded in 2010 with the publication of The Tau Manifesto by Michael Hartl, Tau Day takes place annually on June 28 (6/28 in the American calendar system). The Tau Manifesto’s introduction of $\tau$ (tau) as the “true circle constant” has had a significant impact on geek culture, including changing the time of day MIT