Hi Mandy,
M.N.S.S.H.P is fantastic fun. Basically MK is opened from 7:00 or 7:30p.m. can't quite remember to Midnight The park is all done up in spooky halloween fashion, there are trick or treat stations throughout the park, there are two parades led by the headless horseman which needs to be seen to be believed and most of the rides are open. Lots of people dress up in costumes including the adults and even the characters which is great fun especially for the kids but not a must.
I'm not sure you would have a choice of dates when you are going but I'm sure it will most certainly be happening on the 31st.
We really enjoyed this so much so that it was one of the reasons we decided on going back in October and have definitely planned on doing it again this year.
Annmarie
p.s. if you decide to dress up but don't want to spend a fortune check out Walmarts - that's where we bought our costumes from.