If times you are likely to go are Christmas, spring break and summer, December is actually your ideal use year.
Use year does not affect when you can go (any time of year) or when you can call to reserve (always 11 months out for home resort, 7 at others). Its importance relates to the risk that you might have to cancel a trip and then being able to bank current use year points. You can bank up until end of eighth month of your use year. Thus, what you want to avoid is the possibility of having to cancel a trip that is scheduled to occur in the last few months of your use year because your cancellation date would likely occur after the banking deadline date expires. If you have Dec use year and usually go in Dec, spring break or summer, any cancellation that is likely to occur will occur when you still have time to bank points assuming those trips use current use year* points that can be banked, e.g., cancel in October 2012 a Dec 2012 trip that uses Dec 2012 points and you will have until July 31, 2013 to bank those points or until Nov 30 2013 to use them if you decide not to bank. Even a rtrip as late as August can be cancelled in June and leave you more than a month to bank. Thus, based on what you provided December is your ideal use year.
*Current use year points for a trip are always the points that are issued in the same use year as the trip itself. When you call in Jan 2012 for a Dec 2012 trip and have a Dec use year, the current use year points for that trip that you would use would be ones issued December 1, 2012 for your Dec 2012-13 use year because the trip takes place during that use year. Points from Dec 2011 would need to be banked for that trip.