I don't think that you should ever try to "fit in" your marriage around any sort of schedule. Should you get married in a hurry to be the first (or not the last) of your friends? How about to one-up your brother/sister and be married first? Of course not. If you truly feel ready/committed/whatever (I'm guessing here since I've never been there myself) to get married, then do it!
Of course, it is true that nothing is going to change your life like marriage. AND, nothing is going to change your life like medical school. So, if you suddenly lump these two changes together, you might really freak out at how rapidly different your life becomes in a few short months. If you aren't sure (and by your question, it sounds like you aren't) I would definitely postpone the wedding for a bit. Once you know you are in medical school, see how that affects your relationship. I won't start a flame war about pre-marital cohabitation, so do what you want with that. But, definitely take your first year of medical school as a "trial run" of medicine and marriage. If your relationship suffers somewhat, maybe you need to work harder at balancing the two halves of your life. At least you won't have rushed into marriage and feel the added strain. I would reccommend waiting until the middle or end of medical school to get married. That way your future spouse will have some idea of how much work is required to put up with a future doctor. Good luck in both pursuits!