Guild Wars 2's Halloween event is called Shadow of the Mad King. The event unfolds in four acts. There are also items to purchase.
https://www.guildwars2.com/en/events/halloween-2012
FAQs summary with tips in the comments
http://www.reddit.com/r/Guildwars2/comments/11xmus/faq_for_the_halloween_event/
You need to collect clues for the first act. Here's a guide for part one. The instructions are confusing so the next link also is a guide to part one.
http://www.almarsguides.com/gw2/GettingStarted/Halloween/
http://dulfy.net/2012/10/23/shadow-of-the-mad-king-scavenger-hunt-guide/
This is what the official wiki says, with lists of decorated outposts, lists of quests and items, including headgear, and the NPCs involved in the special event.
http://wiki.guildwars.com/wiki/Halloween
Here is the achievements list in case you collect them.
http://wiki.guildwars2.com/wiki/Special_Event
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