<font color=navy>The Haunted Mansion is made over usually by the first weekend in October, and stays that way through the weekend after January 1st.
It's a Small World gets made over during October. Usually by the first week in November, all of the decorations at Disneyland are up, including the huge tree standing in Town Square on Main Street, and by that time, It's a Small World Christmas is on, Haunted Mansion-Nightmare Before Christmas, and the holiday fireworks and snow are enjoyed. I think California Adventure is decorated after that, and the hotels are usually decorated right before Thanksgiving holiday. The Christmas parade usually debuts on Thanksgiving Day.
The lights go on the trees in Downtown Disney around November, but there aren't too many other decorations until mid-November. (I'm trying to remember) We stayed at the Grand Californian one night in early December, and I remember hearing Christmas music playing in Downtown Disney, so I expect that by Thanksgiving you have all the decorations there, too.
I've been there on the Fridays after Thanksgiving, and I estimate that the crowds there are so huge that maybe you can go on 3 or 4 attractions at Disneyland -- it's wall-to-wall, and shuffle-walking for the most part that weekend.