The party always starts at 7pm, they just allow you in at 4pm.
No party specific events/food/merch will be available until 7. Well they may allow purchase of merch before 7 but will have to show your bracelet to get into the area, that’s what they did last year.
I just wish they changed the theme, Hocus Pocus can only be done so much. I am happy Mickey & Minnie are finally getting the new costumes they deserve.

A-ha! I haven't been to WDW in over 25 years (back when you just stood in a physical line for
all the rides, no exceptions, no extra ride fees, no apps) and this is my first ever
MNSSHP! I'm basically a total Disney newbie in the 21st century.

So the bolded if true answers a question I had: When will they serve party-specific foods like Bucket of Bones and the Snarling Sub? I want to immerse myself in the fun by eating foods with spooky names.

Now, I think it's great that Disney lets party-goers enter the park at 4pm to ride attractions and do other things until the main entertainment and trick-or-treating begins. But for me, if I have to wait three hours to gnaw on Zombie Fingers, I might as well not arrive until between 6pm and 7pm. I have a Park Hopper pass for that week so I already intended to visit MK at rope drop to take advantage of the predicted milder crowds, then have a big early lunch and then return to my Epcot resort to relax/nap until returning to MK for the party. I'm going in October and I'm planning my time well in advance, so whether or not I can get MMMummy Treats 'n' such at 5 o'clock,
will determine how I schedule this day. This is serious!
Our typical party season strategy always involves actually doing MK on a party day we won’t be attending to take advantage of the insanely low waits, and doing the actual party on a separate day. I have rope dropped and properly toured before attending the party the same night—I did it solo last year—but even with a resort break in the middle of the day it was just way too exhausting. I had to totally scrap my plans for the day after bc I was so wiped. I really really don’t recommend it.
In September we plan on doing the tried and true. It’s helped by the fact that there are EEH on our arrival day. We plan on that and the first party day of our trip to take care of our regular MK touring, and will attend the party on our last night.
I'm going for the full jam-packed MK day and night, then sleeping in the next day as late as I please. I'll probably make the day after the party a late afternoon to evening Epcot day because I'll only have to walk over from my hotel. Or just tour the Epcot resort hotels. Or just get a massage and call it a day -- because I might be wiped out!
Another question or two for the pros or anyone who knows: While I'm at MK in the morning of party day, can I get my party wristband then, or do I have to wait until I return in the evening? Is there a deadline to check-in for the party? Meaning, could you waltz through the turnstiles at 9pm? Thanks in advance.