The Gregorian calendar is full of wonderful mathematical quirks, and here's one of them: under its regime, a century will never start on a Sunday. Or a Wednesday or Friday, for that matter.

The reason is (fairly) straightforward. The Gregorian calendar completely cycles every 400 years; add 400 years to any date, and the day of the week will be exactly the same. That means that there are only four different days with which a century can begin, and they happen to be Monday, Saturday, Thursday, or Tuesday (in that that order, for each consecutive century). If that makes your head hurt, watch the video—which explains how the cyclical nature comes about in a little more detail. [Narbis]