The Christmas season is pretty much SDC's busiest time of the year, save for perhaps MM and the other holiday weekends (if the weather's good). However, this shouldn't dissuade anyone from going. The whole park is an experience to enjoy, and many of the crowds are wrapped up in viewing the sites, the shows, the food, and other things, so unless you are trying to see all the shows it's not too difficult to do a lot even with a full park.