Oct is traditionally a good time. The weather is good, the crowds are low. Last year was really, super busy because of the 50th.
Dec has some good times, and some super busy times. Early in the month is pretty good. The week before Christmas is good. The week between Christmas and New Years is crazy busy. Christmas time in the park is really magical. Of the 3 times, this probably has the longest park hours, the biggest crowds and the fullest entertainment schedules.
Jan. is pretty quiet, but there are typically a few rides and things closed for refurb. Park hours are usually shorter during this time of year, and the entertainment schedules are pretty scaled back.