They have Halloween Spooktacular every year where kids can come in costume and trick or treat etc. I am sure the 31st will be one of the days but don't know how crowded the park gets.
We are possibly going the end of sept/early oct so your post made me wonder about us seeing any halloween events for our trip. I googled and found this. It's 2008 informatoin but maybe they will do something simliar this year?
They have some trick or treat for kids. Well, adults can do it to, and the candy is really nice.
A part of the park is decorated. It's not a big celebration but it's a nice way to get in the halloween mood.