As others have already said, yes it's a good idea to exercise leading up to (and it goes without saying, after) Christmas! The psychological benefits have already been mentioned but they shouldn't be ignored. The average person gains two pounds over the holiday season. If working out keeps you from gaining those extra two pounds, then you'll be much better off. Also, working out isn't an all or nothing thing. You can take it easy over the holidays as family visits, cooking, parties, etc. will likely have you busier than usual. Cutting it out completely though may make it all that more difficult to start up again when New Years rolls around.