First, I haven't been to
Disneyland in a long time that HASN'T been during October... with that said...
I'm surprised everyone is saying crowds are so horrible during October. I certainly think there are really really high traffic days, but overall I've REALLY enjoyed my October trips the last four years or so. What I've personally noticed is that obviously weekends are rough, but don't get that way until about noon, so if you're staying onsite or close by and get in right away in the morning, it won't be too bad for your first three or four big rides. Tuesday Halloween party days also don't seem bad, this year will be interesting since it's not in Disneyland, but previous years it seemed to be very manageable until about 4pm in the afternoon, then by about 6 or 7pm the Halloween crowds didn't seem too bad, other then the fireworks and parade. Week days didn't seem too bad either.
My trips are usually as close to Halloween as possible, without actually being down there during Halloween, and usually always included a Sunday. This year's plan is Oct 19-24th (24th will be leaving Disneyland at about 4pm to go do Knott's Scary Farm). Last year was 20th to the 24th (20th, Saturday, arrival, not in the park. Sunday, Monday, Tuesday full park days with Halloween party on Tue, Wed was half a day before Knott's Scary Farm). Previous year would be the same kind of pattern.
I think the time of year and the overall feel of the theme parks certainly balances out any extra crowds, as long as you can spend at least a two or three (or more) days there and get to the park early, at open or within an hour of open. The weather is also about the best you're going to get before it moves into Christmas season at the parks. A couple times it broke into the 90s, but usually 78 to 88, which is a little high, but only during the late afternoons.