I cancelled my service and Xfinity wants to charge me for the next month.

I planned for my service to be canceled on December 16 the end of my service and am being charge for the next month in which i shouldn't have service. I see they do this to a ton of people as this forum is full of people complaining about Xfinity charging people without service. I even have the work order number that they gave me from that phone call: 1002095166902029