Simple Zeller's congruence to get day of week

[TDarcos]

I have edited the first post of this thread to warn people reading the material later that the formula I'm using for Zeller's congruence has some error in it. Beyond that, I made no other changes. As I point out in the warning, I take responsibility for my actions, whether the formula was wrong or I copied it wrong, is irrelevant. It also means you don't hide your mistakes, you learn from them.