Hi Laura!
Mickey's Halloween Party is one of the special event parties held at Disneyland each fall. This year guests can expect much more with more areas being decorated and the addition of the California Adventure Park being open for ticket holders as well.
Disneyland Park is certainly open for fun and magic that night. Literally almost every attraction is open for guests to enjoy. The list is quite long so it is easier to talk about what is not open. Attractions that are affected by darkness will be closed. These will include things like the Railroad,canoes, riverboat, Columbia ship, and Tarzan's Treehouse. Much of Toontown is also closed although there is usually trick or treating locations there.
The highlight of the party is certainly the parade and fireworks and the recently added ride of the Headless Horseman down the streets of the park.There are also three attractions that take on a new look during this holiday time and not not to be missed. Those three include "It's a Small World", the "Haunted Mansion", and "Space Mountain".
With almost every ride operating and the trick or treat stations as well as the great entertainment this is a night to remember for years to come when you head to Disneyland at Halloween!