They start decorating sometime in November--I don't remember the exact dates, but the Christmas season is basically in full swing the week before Thanksgiving (which I read was an excellent time to go this year, by the way).
I personally found the Wednesday - Sunday (afternoon) after the Cast Member parties (which are generally the first Monday and Tuesday in December) to be delightful times too, with Fastpass hardly even being useful--but Friday and Saturday would have likely been much busier if it hadn't rained. Still, entertainment was almost in full-swing by then (with Fantasmic running on Wednesday and Thursday, and evidently one show on Friday; the second and Saturday's shows, and some parades in that period, were rained out

).
Disneyland park was open 9 - 11 on Wednesday and Thursday and 8 - midnight on Friday-Sunday. Crowds were so light overall that we generally got there at opening (opened up DCA on Saturday) and then stayed until we got tired, with no mid-afternoon break. The cool weather and lighter crowds allowed us to pull as much as 14 hour days without killing ourselves.
The only thing that wasn't offered when we went was the Electrical Parade, which only runs during the busiest part of the holiday season (about the last two weeks). For me, lower crowds throughout was more than worth the tradeoff of no Electrical Parade, but if that parade is a priority for you and your schedule is otherwise open, you'd probably want to time your trip to end on the 17th - 19th of December, so you get lighter weekday crowds (the week directly before Christmas will likely be pretty busy) but still get to see the parade.
(Of course, this is all assuming that the Electrical Parade runs next year in the exact same way it did this year, which may not be the case, as there's no way of knowing.

)