Probably a 'premium' date is where a lot of people were there historically.
In November, this year's premium dates are:
Nov. 14 and Nov. 21.
The only thing that I can see that 11/14 and 11/21 have in common are that they're Friday nights... typically a 'crowded' night in the parks if I'm not misteaken.
As far as I know, the difference between a premium night and a non-premium night is that a ticket costs more money. There are no additional features in the parks, but I could be wrong... at least I didn't see any when we were there.