On October 15 between 2-5pm your local time, the Litwick will spawn in large numbers and you should be able to get a shiny Litwick easily. Shiny, whose flames are blue rather than purple, is actually the perfect Halloween companion Pokémon due to the beautiful orange color it acquires when fully evolved.
During the event period, XP from catching Pokémon will be tripled and candy from catching Pokémon will be doubled. The annual Pokémon Go Halloween event also typically doubles candy catches (as it often does for Halloween), but we’re not sure if this overlaps with Community Day or a potential candy reward will be stacked.
Additionally, evolving a Spotlight (Litwick’s evolution) into a Chandelier from 2-10pm local time will teach it the charged attack “Poltergeist”. It’s a bit early to decide if this move will be meta-determining, but Chandelure is already a very strong Pokémon in both the ghost and fire genres, so you should definitely plan to stock up regardless.
Lights require a Unova Stone to evolve into Chandelure, so you should try to save whatever you have until the event occurs. Previously, Niantic added short research quests that rewarded them so you wouldn’t have a hard time not being able to evolve your creature, although we can’t confirm if the game will feature a quest for Litwick Community Day or not.
Pokémon Go players have been waiting for this adorable candle Pokémon to get its attention, as it fits the basic criteria of a Community Day creature: it evolves twice and is powerful.