December is my slowest month. The rhythms of the academic year are fairly predictable, and the December trough is consistent: everyone wraps up their articles and chapters and dissertation drafts by Thanksgiving, and whatever isn't done by then won't get done until January.
I remember the craziness of the end of the semester (student freakouts! Grading! Finishing departmental reports! Looking at the dreaded Job List!) I also remember the absolute collapse into catatonia at the end of the semester. After grades were turned in, I always felt like I was physically incapable of doing anything except drinking eggnog and sleeping late and wrapping presents and reading cozy English village mysteries to rest my poor tired brain.
None of us are brains on sticks (and we shouldn't be expected to be). If your body is telling you to take a break—a real break—at the end of the semester, please listen to it. Self-care is a good thing. Rest your body and your brain.
The deadlines and journals and job applications and committees will still be there waiting come January.