The Halloween Events take place in Queensdale, Kessex Hills and Gendarran Fields. These events work like all the events of Guild Wars 2, they're active and available for all players to participate in whenever they pop up. The events are almost always going on in at least one of the zones and when they aren't a ton of Haunted Doors will be up around the entire zone that will transport you to the Mad King'd Realm where you can access the dreaded Clocktower and Labyrinth.
Below are pictures of where you can find all the different events in each of the three aforementioned zones. Keep in mind that if a picture is too hard for you to view because of it's size click on it to enlarge it.
Queensdale Halloween Event Locations
Kessex Hills Event Locations
Gendarran Fields Event Locations
The blue circles mark all events that currently are not up on the map. Also note that every single event in this zone for Halloween is a Group Event. They will be next to impossible to complete solo.