We went into MK early last year on the day of our party and stayed all day; it was just a bit too much for me. I ususally have a lot of energy, but walking around all day and then staying for the party...well, I was on overload.

Another thing to think about is what all you plan to do at the party. If you spend time getting treats, watching parades (which you don't want to miss), meeting special characters that only come out for the parties, and doing some of the special events you really won't have much time for rides. Also, only a certain number of rides are operational during the parties.

If you spend the whole day at MK (like we did) I would suggest that you take some break times just to rest up for the evening party hours.