The entire town is getting a spooky (and pumpkiny) makeover when the event goes live with Game Update 19. It's not all decorations, of course; there are plenty of activities to experience as well. The main villain of the event is Klarion the Witch Boy, who is stealing powers and using them for... mild mischief. But he's not all bad, since he's throwing a massive party that players will have to crash.
SOE has plenty of rewards for good little trick-or-treaters, such as trinkets and outfits. The Witching Hour is scheduled for early October.