Tables are assigned on the moment you call for your ADR.
Once at the Poly, you have to go to guest relations to get your tickets. I'm sure you can do this some time ahead.
We had ADRs for the 5.15 show. We were at the Poly at 4, and by the time I had figured out where to go/ what to do/ how to get the tickets, we still had to wiat a very long time in the lobby. While waiting, your picture is taken with tiki things.
A short time before the show, everyone goes in group to the Luau Cove, and there you wait until you get seated, but tables are mentioned on your tickets, so it doesn't matter if you're there the first one or the last one.
I think dinner was over by 7 pm.