We haven't been to
Disneyland during Columbus day week since 2005, but we were shocked how busy it was (Friday before-Tuesday). It was during the 50th anniversary celebration, so that could have had some impact on the crowds. This year Columbus day weekend coincides with Gay Days (5th-7th), so I would expect the weekend to be busy. Not sure how the rest of the week will be. Since the introduction of "Halloweentime", October has become quite a busy time to visit.
UT and AZ school fall breaks are later in the month, so that will help with the crowds.
We have gone to Disneyland in October every year for the past 6 years, and it has rained every trip. No matter which week of the month it was. Usually in the evening. We have been rained on at every Halloween party since 2007! You shouldn't need more than a light jacket in the morning and evening (and rain ponchos).