Well, there is seeing the Seeing Christmas decorations...and then there is getting the full
Disneyland Resort holiday experience.
The holiday season won't officially begin at DLR this year until either 11/11/11 or 11/18/11, and when it officially begins is when you get to see the gorgeous It's a Small World Holiday, the Winter Castle glowing in "icicles" at night, the snow on Main Street, the Christmas Fantasy Parade, Santa's Reindeer Round-Up, all kinds of holiday treats and all the stuff that makes the season magical.
And after Thanksgiving passes is when the 3 Disneyland Resort hotels are decorated too (the Grand Californian is a highlight).
So if you want the full holiday experience with all of the entertainment and decorations in different lands and everything else that comes with it - you won't get that until at least 11/11, if not 11/18. But you
will see Christmas decorations going up a little at a time if you get there before the official start date.
If you go at the end of October you will catch the HalloweenTime season while it's in full swing. It's not as all-encompassing and thorough as the Christmas season is but it's a lot of fun.
I think the early November crowds might actually be lower than the late October crowds, so you may want to go with that time frame after 11/4.