It's a sorta once-only thing for their 25th anniversary, so you won't find anyone who's ever been to it before.
Basically, it's a private party, the entire park, attractions, restaurants, etc., is open for those with tickets. It's like Grad Nite, except with saner people , and with more things open. There won't be any fireworks or the like, of course (I think)
KOST might do something themselves during the event (they'll certainly be broadcasting, probably from in front of the castle), but there isn't anything especially extraordinary being arranged by DLR itself, at least that I know of.