October is tricky, because Utah and Arizona schools often take "fall break" during the first couple of weeks. This year, the Utah Education Association conference is early, Thursday Oct. 1-through the weekend, so all of Utah will descend on Anaheim that weekend. Also, Gay Days is the 2nd-4th, which adds a lot of people. So I'd definitely avoid that first weekend.
Actually, I'd avoid weekends, anyway, because they are always busy, and especially in October.
Disneyland has their wonderful Halloween decorations up, and they have Mickey's Trick or Treat party over in California Adventure on the weekends.
You might want to check and see when the big Arizona districts take fall break. I think I've seen a lot of them are going to be the week of the 12th-16th, which is also Columbus Day Monday.
So possibly your best bet would be Mon-Friday, the 6th-10th? You probably will miss fireworks, though, unless you stay to see them on Friday. And in October, the Santa Ana winds are blowing, and they can be canceled because of "winds at higher elevations" sometimes. We were there last year in October, and saw them on Sunday before Columbus Day, but on Columbus Day, they were canceled due to the wind.
Hope that helps.