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]