I'm starting July 12 - I want to be done by Christmas, by Thanksgiving if at all possible. But December in the Smokies is not that bad. I did about 66 miles last December in the northern half of the park. There was rain and a little snow and ice. Some very cold nights. The most aggravating part is the fact that there is so little daylight to hike in. December days are short, and the Smokies are pretty far south, so I always feel like I have to be up before dawn and hike until dark to cover any distance. So there is a lot of time in camp in the dark. Nights can be long and cold.
But the scenery is fantastic, with the leaves off the trees so there is excellent visibility. Sunrise and sunset in frost and snow is breathtaking. The wind howls through the trees like a freight train. Getting out of the warm sleeping bag in the morning can be a daunting task, and don't get me started on putting on frozen shoes! But as long as you have the right clothes and sleeping gear, and are willing to put up with some discomfort, no problems! Springer never "closes", so no deadlines...