The Halloween event started on October 22nd, 2017 and ended on 5th November, 2017 and featured 1 new Halloween pokemon and a returning one, The event was set to release on October 21st, 2017, but was delayed by one day.
Chompy made its return from 2015, the way you obtain Chompy is exactly to how you got it originally in 2015, by finding it randomly on the map, the only difference being it doesn't spawn as much as it did in 2015.
Skulltle is a new event pokemon that was released during the event. It is a skeleton Squirtle that is a water/ghost type. The way it is obtained Skulltle is by collecting 100 bones during the event. Bones are found by either collecting them around the map, or by earning them by killing other players.
Misc and other stuff
Bones were not for only getting Skulltle, after you obtained Skulltle every 5 bones afterward rewarded you with a rare candy.
Along with the Halloween update some other pokemon where added such as
The item, Reaper Cloth, was also added.
During the event, it is always night with no possible way to change the weather, the Moon being a Blood Moon and your POV shortened by thick fog.