A Disneyland ticket is not required to go to the Halloween Party. If you have a party ticket, you get entry into Disneyland on the day of your party AFTER 4pm.
As it is Disney probably going on all weekend or week but you could check their website to be safe!
Mickey's Halloween Party is a separately ticketed event. It would be unfair to guests of the Halloween Party if regular guests were able to have the same experience!