Mt. Tremblant can be great but I would hesitate to take a big trip there in December. There may not be much snow yet. And it can get very cold and windy all winter. I prefer the spring. Plus it could be a good snow season or a bad one.
If you are a picky skier like me(decent snow, sunny skies, reasonable temps, less crowds), then Tremblant is like much of eastern North America....a last minute destination after checking conditions out a couple of days before.
Tremblant in Winter
Two megastorms just hit Mt. tremblent and Northeast US. Now just wait for a nice day.