Hey, Anonymous. Just had to comment that a lot of us feel that way (I'm dealing with the post-Christmas depression problem right now). Best I can figure, it has a lot to do with weeks of building up for Christmas and then -- bam! -- it's over in one day. As soon as we cross midnight on Dec. 25th, it's rather like we simply shrug our shoulders and get back to work.
It wasn't that way when we were kids, though. No, we had a couple of weeks off and Christmas was more like a cherished season than a day off from a job we hate.
The point is this -- returning to "normal" after a great holiday is tough for some of us adults who love the break from reality that Christmas affords us. I do believe, then, post-holiday depression has a lot to do with the fact that Christmas gives us an escape from a lot of unpleasantness, and that escape is over when the holiday is gone.