Microsoft is offering 3.14% off Dell tablets today, notes Time. The Your Pie pizza chain is offering pizzas for $3.14 all day in the Southeast. And Caltech students began a pie party at 1:59am featuring 130 pies—26 varieties of five each. Which, as the Pasadena Sun points out, makes use of the digits 3.14159265. It is, of course, Pi Day, a celebration of that all-important number in math—possibly "the most important single number in the entire cosmos," writes Phil Plait at Slate, who offers a detailed explanation on why that's the case. It also happens to be Alfred Einstein's birthday.