The world of Guild Wars 2 is getting ready for Halloween with a major content update coming on October 22. "Be prepared for an epic adventure in four acts", running until October 31, ArenaNET says on this page detailing the update.
Halloween decorations, mini-games and dynamic events will accompany "Shadow of the Mad King," the first major game release for Guild Wars 2. From its description, it sounds like a ghost hunt, for the 250-year-old spirit of Mad King Thorn.
The Black Lion Trading Company also is stocking Halloween costumes plus other new items, PvP paid tournaments (which you may read more about here) also arrive, and a broad, non-Halloween themed update bringing "new events, jumping puzzles, bosses, mini-dungeons, and achievements appearing across a variety of maps," is also included.
Halloween 2012 [Guild Wars Official Site]