15th October, 1582
The 15th of October in 1582 was a very special day. For fun, try typing it into iCal on your mac. (From the View menu select “Go To Date…”, and enter in 15/10/1582). If you’re in month view, you’ll find it’ll show you September, an no matter how hard you try, you won’t be able to find October of 1582! If you go to week view however, you’ll find October, but something will be wrong…
There’s 10 days missing!
I can’t really explain it all, but there’s a pretty interesting article on the Panic Blog that does it pretty well.