Well the dessert parties for our December trip finally opened up and are already all booked. I will keep checking to hopefully catch a cancellation but need a backup plan just in case.
Where is everyone’s favorite spot to stake out and how early before they start do you try to get there?
Which park are you talking about? Or all three of them (MK, HS, EP)?
nothing beats seeing MK fireworks from the Main Street Rail Road station - IMHO
Any, this is our first trip to WDW but are DLR Pros 
On our recent trip, we had good views of the various fireworks, without waiting too long, but certainly they won’t satisfy everyone.
MK: sitting on benches on porch of Crystal Palace (some small trees block view of part of castle, so not good for CTM), or standing in the middle of the thick crowd in the hub where Main Street joins the hub (good for adults only, not good for kids). We just worked our way into place about 10-20 minutes before it started, CL 7 and 10.
HS: standing between/slightly ahead of two big projection towers in area in front of Great Movie Ride. You need to be at least that far forward so towers don’t block views of all the buildings they do projections on, plus you hear the surround sound all around you. We got there maybe 20 minutes before it started. CL 1.
EP: one spot was to the right of the Canada’s big boulders along the shore, to the right of a tree and in front of a raised flower bed. Good view, could rest against raised flower bed, and somewhat near International Gateway for easy exit. I got there about 20 minutes before it started. CL 6.
The second time, we watched from where the meandering sidewalk near Journey into Imagination meets the promenade around the lake. We could rest against a railing, then when it was done we had a short walk to Soarin’ for EMH ride, and to the front entry. We got there about 15-20 minutes before it started. CL 8.
We like to lean against the fence behind hub garden west, in front of the baby care center.