We went to the party next to the last last year. Crowds were manageable...the longest we ever waited in line was 25 minutes for Thunder Mountain. Parade was VERY crowded though so getting a decent seat will take some planning. If you don't care for the parade, then just go on the rides during.
I haven't had the time to scroll through all the responses, but just in case you didnt know this already, you can get into the park as early as 4:00 with your wristband. Definitely do that!
As far as which day to go? We went towards the end of our trip because we wanted to close out the trip that way and I'm glad we did. It leaves you with that "Christmasy" feeling, the lights at night, etc. Our whole trip was kinda building up to that night with the cookies and cocoa, etc. We really enjoyed themselves.
And as far as hanging around to decide if you want to buy, that might be a good idea of how crowded it might be. I know when we went last year, we bought out tickets the week of our arrival and that was the last week they were having the parties...so most likely you'll be alright, but you do take a slight risk in not having pre-ordered the tickets.
I would honestly pick which day you wanna go and order advance tickets. (And as far as discounts, I don't know of any except the discounted military tix we got last year. But you can't pre-order those, etc.)
Also, you'll see alot of people wearing Santa hats, etc. and we've decided the next time we get to go to a
MVMCP, we're gonna do that too! It just seemed all the more festive!
Lastly, if you plan on buying party-related souvenirs, pick those out early in the day. Even better, if you are staying on property, buy your stuff and have them delivered to your resort instead of carrying them around. I missed out on some stuff last year because I waited too long in the night to get my souvenirs and they'd already sold out. Again, this might not be the case for you since you're going earlier in December.