December 6th. You call 11-months from what will be your check-OUT date, so staying the night of Nov 5th, you would be checking out Nov 6th, so you can call 11-months earlier on Dec 6th.
In this case Dec 6th, 2005 is a Tuesday so you're OK. Should the date you want to call fall on a weekend or holiday when MS is closed, you have to wait until the next day they're open. You can't call earlier.
EG, suppose Dec 6th was a Saturday, then you'd have to wait till Monday to call.