Tony, although GAdv is my home park, that's the first I've heard of the zombie thing. I don't know why they'd do it almost two months before Fright Fest begins. Maybe it's tied in with a movie or something. I did see where another park did a an event with the same name, but that was tied in with the Coaster Zombies roller coaster club having an event. Maybe they're doing something at GAdv?
*EDIT*..from GAdv's site:
Saturday, August 4, 2012 the NJ Zombie Walk presents Six Flags Zombie Takeover! Event includes: zombie parade, costume contest and exclusive ride time after the park closes. This is your chance to audition for Fright Fest in our 1st ever "Z" Factor Talent Show! More details and discounts available soon.
So I guess it's just a one-off deal, to do something different during the dog days of August.
