We are waiting on dates for Mickey's Halloween Party, but this post (from the Halloween Time at DLR Superthread) will give you an idea of when the party took place last year (look at the bold red text), in case that influences your trip dates -
http://www.disboards.com/threads/ha...ut-halloween-time-2015.3270919/#post-51326911. I will update that post when the 2015 dates come out, which will be very soon. Feel free to join us in the Superthread any time!
I think that Geemo's idea of somewhere between 10/18 and 10/25 is a good guess as to which would be the best week. October is technically considered to be the off peak season by
Disneyland, but to guests in the parks it doesn't seem slow at all. Halloween Time is a popular season to go, and unless there is rain happening throughout the month it will probably be pretty busy most of the time, especially in light of the Diamond Celebration that is currently happening.
Also keep an eye on this Unofficial Park Days page for unofficial, lesser-known events and special days that may pop up, bringing lots of extra people to the parks -
http://unofficialparkdays.com/calen...~1448006399/cat_ids~69,6/request_format~html/. These are not official Disney events, but just random events for other groups who visit Disneyland (such as Christmas in July Day). I don't think the calendar is fully filled in yet, as there appear to be no special "days" happening in October so far, but I'm sure it will be updated.