Keep in mind that failing a student during his/her defense is an immense embarrassment for the advisor, who surely wants to prevent exposing him/herself to such situation. I have heard of advisors forbidding their students to defend, they were lound and clear. The only situation close to a student defending a failing thesis I've ever hear about happened when his advisor had some very serious personal problems and did a very lame advising job. Even in that case, a member of the committee readily pointed out well in advance for the defense that the thesis had serious flaws, and thus the defense was postponed.
A few alternatives you might consider to explain the situation:
Perhaps, your advisor is very busy by the end of the year, and is eager to postpone stuff. Often people will accumulate tasks by December.
Maybe your thesis is okay, but you likely need to prepare slides for your defense, you need to rehearse your defense, and you need to rehash it a few times. If you are not that used to public speaking and slide preparation, your advisor is should instruct you to do so. He/she should be more open about it, but it mightcould come across a bit awkward to tell you that "your thesis is good, but your presentation is not ready yet".
Maybe, other than the actual thesis work, you are also responsible for doing lab work/preparing classes, grading exams and so on. If you defend in December rather than in March, maybe your advisor will have no one to perform these tasks for him. Very petty, and only his/her problem, but I'd find it very plausible that an advisor in this situation would try to stall the student's defense without giving clear reasons.