Gengar and Sableye are receiving Halloween costumes. You can find the former’s in raids, and the latter either in eggs or by encountering them in a wild battle. Spiritomb can be found through Normal Research, and you’ll have a chance of getting a Shiny one. Meanwhile, some Special Research tasks called “A Spooky Message Unmasked” can get you Galarian Yamask, and you can get Mega Gengar through Timed Research by collecting Gengar Mega Energy.
As for other ways to celebrate:
- Alolan Marowak Raid Day — On Saturday, October 31, 2020, from 11:00 a.m. to 5:00 p.m. local time, Alolan Marowak will be appearing more often in raids.
- Catch Mastery: Ghost Day — On Sunday, October 25, 2020, from 8:00 a.m. to 10:00 p.m. local time, Ghost-type Pokémon will be appearing more often in the wild and there will be event-exclusive Timed Research available to complete.
- GO Battle League: Halloween Cup — From Monday, October 26, 2020, at 1:00 p.m. PDT (GMT −7) to Tuesday, November 3, 2020, at 1:00 p.m. PST (GMT −8), the Halloween Cup will be live in the GO Battle League. In this format, only Poison-, Bug-, Ghost-, Dark-, and Fairy-type Pokémon with 1500 CP or less will be allowed to participate. You can also earn encounters with costumed Bulbasaur, Charmander, and Squirtle.