UMB's winter semester is optional. many people use it for electives but you don't have to attend at all. If you don't attend, you will have a break from about the 21st of Dec (whenever your last final is) to the end of Jan (23?). The other thing to remember is that rotations come into play-first year's have a one day rotation (usually scheduled over winter break or over spring break) and of course 3rd years +4th years might be doing rotations over that time anyway.