It's not the same for professors, I think, because we see each beginning of semester as a chance to start again. A class didn't quite work as your thought it would? We can try again with some modifications! Didn't like the classes you taught last semester? Here is a new one to keep your interest! Didn't like your students? A fresh batch is waiting! Not to mention that Decembers ends with hours of grading, grade grubbing, and last-minute administrative work.The first few lectures are usually well-worn territory as well, so prep time might not be as stressful. And we don't know the students yet. There is something invigorating about the potential that each new class brings to the table. At least for me, the excitement of the beginning of the semester is rarely diminished. Especially when a class works well, I can't wait to get back in there with a new group to do it again.
But the students, well, they don't see it that way. Maybe they went home at Christmas and realized just how homesick they really are. Or they got sick from the cold/lack of proper sleep and care and start off the semester feeling terrible physically. Maybe they hate their roommate and so are coming back to a lot of drama that they just didn't know existed in the fall. Or their semester didn't go as planned and they are bringing a defeatist attitude to the new semester.
Or all of these freaking snow days are making it hard to get into a good routine. No wait, that's me.
In August and September, the weather is nice, the days are long(er) and the mood is full of optimism. In January, the days are colder, shorter and gloomier (even in California - I taught there). Two weeks sometimes is just enough time to remind you why you love your family but not enough time to remind you why you left for college to begin with. It's really not enough time to put any disaster, both personal and academic, behind you to be able to really start fresh.
I'm not arguing for a longer winter break; we started really late this January, and while I welcomed the time off, I am not looking forward to teaching until mid-May. But I have to continually reminded myself of the reasons behind the January Blahs when my students keep looking at me like I'm crazy with my enthusiasm and energy. I didn't get those looks (or at least not as many) in the fall. It's a bigger challenge in January to get my students to buy in to what I am selling them. It just means I have to work a little harder and remember where they are coming from.