I have been skiing in Tremblant every Xmas for over 20 years. It is better than anywhere in the Laurentians and possibly Eastern township and Ontario. While you may not be able to ski the most difficult runs and the glades for lack of natural snow during that time, both north and south and often the Soleil side will be open. There is lots of activities and festive atmosphere. I would recommend it if you do not mind a slightly higher price there during Xmas than any other place in Eastern Canada. The other option is Mont Saint Anne. But you have to stay in Quebec city for the festivities and not as convenient as Tremblant.
Depending on when you go you can get the cheap tickets by buying before the Dec 4 deadline I think it is 49 for time outside Dec 26 or 27 - Jan 1 (Check website for detail) but around 60 something during the highest seasson on Dec 27 28 29 etc. It is called the Latitude Card. you also can get cheap tickets by staying at the hotel there and order ticket from them.