My trips are in August, so I tend to pick based on the day of the week. If I can go on a Tuesday, I pick that over Friday. I try to avoid the 1st Halloween party simply because I prefer to have all the info about the party's offerings -- snacks, Magic shots, entertainment times, character locations and greetings etc. If I had a choice, I'd rather go on my last night than my arrival night. That being said, I'm going to two parties - the 2nd party (Tuesday) and the 3rd party (Friday) which are nights 3 and 6 of my 9 night trip.