You call 7 months from check OUT, not in, and MS will not be open on Christmas Day. If the 7 month window falls on a weekend, you must wait until the following Monday morning to call.
You may want to go to www.caskbill.com and download the FREEDVC Planner software that Bill's written. It gives you the points for all the resorts and seasons, banking deadlines, booking windows for both 11 and 7 month windows, and a ton of other useful stuff. The calling dates are even adjusted for days when MS is not open!
It's a really essential tool. I don't think I'd ever want to plan a trip again without it.
Or you can go to the DVC Member web site and look under My Points. It will tell you up at the top of the page when you can use your points to stay, at your home resort and at another non-home resort.