https://www.gamespot.com/articles/all-the-halloween-2017-in-game-events-and-updates-/1100-6453986/

One thing you can count on every October, aside from a bunch of big game releases, are developers releasing Halloween-themed updates and events. Halloween is still weeks away, and already we've seen a number of such announcements, and some of these events are already underway. [Updated October 19 with official Pokemon Go details.]