When I get back with my students after the Christmas break, I like to hear what they did with their two weeks of freedom.
The most common response is, "It went by too quickly" or "It wasn't enough time." Etc., etc.
That pretty much describes every generation of students that ever was. A two-week respite from the salt mines of the classroom is, was and always will be a pure pleasure, greater even than the three months off in summer. Back-to-school in September came after three months off. After that, there was a general readiness to return. The two weeks in December are different. They come toward the end, and thus are an interruption of, the first semester. It always makes starting up again after this recess that much more difficult.
If we had started school in mid-August, this winter vacation would come at the end of the semester, making it a more natural break. There are a number of sound educational reasons to reconfigure the school calendar to start earlier, but it won't happen. Why it won't happen is a question that gets no good answer from the people who set policy.