Halloween takes over the Zombies GameMode. Players have randomised Halloween vanity and mob waves are replaced with their Halloween versions.
Whether this event applies to normal zombies or as a separate joinable variant with a specific map is dependent on whether a map with waves is ready in time or not. @Geolindrag is working on such a map.
~ TBD ~
Posting early so people are aware of the dates that the event will run over. Also be aware that any quests will stop being completable the moment the event ends, even if you are still playing in a lobby.