It's just one Christmas. Skip the parties and don't worry about it. Saying 'No' to things that aren't good for us or that we simply don't want to do is one of the great advantages of being an adult.
Next year will come soon enough and when you have a little more sober time, you'll be in a better place to evaluate whether you simply have no interest in those types of parties anymore of if you want to go just for the social aspect. It's not like you're saying that you'll never go to a Christmas party again, just that you're passing on them for the next couple of weeks. In the mean time, find a sober way to get in the holiday spirit- hang up some lights, bake some cookies, make some fudge, buy some presents for your family, listen to some Christmas music.
Sobriety comes first and if you have a bad feeling about going listen to that. Nothing good could come of going to an event that you don't really want to go to anyway.
Another thought is just that recovery isn't just a straight path. Some days and weeks things will click more easily and you'll feel like doing those things like going to the gym. Other days, getting out of bed might be a challenge. As long as you don't drink, the number of good days will grow and the bad days will become less frequent.
A few days or a week feeling kind of unmotivated is fine. If it goes on for much longer, that is time to start kicking yourself in the butt and doing positive stuff again.