I've gone in September (first night of party or first weekend), mid October and Halloween Night. One year I went twice - September and a few nights before Halloween. Once the times are set, they've not changed. Unless they've added the projections from HEA, I can't see the times for the shows, parades and fireworks changing from 1st night this week to the last night November 1st. They'd have to reprint the party guide they hand you when you enter. And we all know TWDC doesn't like to spend money it doesn't have to...
I'm psyched about going next Friday, mainly because none of the party specific merchandise will be sold out. However, it's going to be hot, hot, hot....
Tip, if you've not attended before, the treat bag they hand you is this tiny plastic thing. Disney does the treat thing right. Several spots around MK (just look for the jack o lantern sign), BIG barrels of the good stuff, and they give it out by the handfuls. And you can visit each spot several times - I've never known them to run out during a party night. I'd visit Publix before you leave for WDW and get one of their reusable Halloween bags (ones similar to the reusable grocery bag).
I'm bringing this next week:
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It will hold all my candy, any purchases I make and my ears when they start hurting my head.