The other nights are rehearsals, and everybody involved knows it. The park knows it. The veterans who go multiple nights a season know it. The ticket price on the last one says it out loud.
On actual Halloween the party and the holiday finally line up. The Headless Horseman rides through a park on the one night of the year the calendar agrees with him. Madame Leota presides over a street seance while the rest of the country is answering the door with a bowl of candy. The Sanderson Sisters materialize on the date they are owed. The atmosphere the park has been building since late summer finally has a real Halloween to meet, and the Treat Trails run right up to the end of it.
California Adventure in Anaheim, a separate ticket from park admission, the party running late into the night.
This date goes first every year. Decide now who you are going with.