The biggest mistake that visitors make when visiting Disneyland in October is not understanding the difference between Halloween Time and the extra ticketed Halloween Party, Oogie Boogie Bash.
The Oogie Boogie Bash Halloween Party happens on select nights and is not included with your park admission but instead is a separate party ticket that must be purchased.
To maximize your time in the parks during the Halloween season, I recommend you plan on starting early, taking mid-day breaks if needed, and going back in the evenings.