Basically it consists of 4 acts, with a special one-time event happening on October 28th at 12pm PST (3pm Eastern, 4:30pm Newfoundland time)
So far, the activities available seem to be:
Costume Brawl (check out around the mystic forge in Lion's Arch)
History of the Mad King scavenger hunt (The reward for this is the Book back armor in the image above. If you need help with this, myself and the officers have already completed it, and would gladly help you find all the pieces!)
Haunted Doors (randomly spawning in Queens Dale, Kessex Hills and Gendarran Fields)
More activities and events are bound to crop up as we make it through the acts, so make sure you hop online to complete them with us!