Halloween has always been a merry time in Guild Wars, with the Mad King Thorn visiting the capital cities of Tyria to dole out tricks and treats. It seems he'll be returning in Guild Wars 2 as part of "the first major game release" for ArenaNet's subscription-free MMORPG, which shall also bring paid PvP tournaments and gobs of new stuff to see, do and kill.
The Halloween festivities will kick off when the Shadow of the Mad King update drops next Monday, October 22. Except spooky decorations, macabre mini-games and dastardly dynamic events, taking place over four 'acts' until October 31. Halloween costumes and goods will be on sale at the Black Lion Trading Company too.
If you want serious business, you'll be able to get into five-person paid tournaments by winning normal tournaments or buying tickets in the Gem Store. These will give better prizes than regular rumbles, and you can earn qualifier points to compete in upcoming larger tournaments.
As for plain old new things, ArenaNet says the update will see "new events, jumping puzzles, bosses, mini-dungeons, and achievements appearing across a variety of maps."
Do check the event page for more information.