This is Halloween is an Odyssey Event from Monday October 23rd, 2012 @ 8:00pm (pst) to October 31st 2012 @ 11:59pm (pst)
Odyssey Events are limited time events that consist of 20 chapters of Quests similar to the normal Quests. The chapters are grouped in sections of three with a boss at the end of every 5 quests. At Chapters 10 and 20, special bosses are fought that reward the player with a special reward card.
Odyssey Events also have special Treasure sets that reward the player with a special card upon completion.
Sometimes, special items can be won in the Odyssey Events which help to place the player higher in the rankings at the completion of the event, such as Stray Souls in Creatures of the Night, Forget-me-nots in Dead Memories, and Stellar Fragments in Stargazer.