Developer Niantic Labs and The Pokemon Company announced In Game for Halloween Special this week. The event runs October 26 through November 1. During that time, some of the game's "spookiest" creatures will show up more often, including Drowzee, Gastly, Gengar, Golbat, Haunter, Hypno, and Zubat.
True enough, I caught all the Gastlys and Haunters in my neighbourhood on Friday. Wow, I can't imagine how many of these spooky Pokemons will be spawned on Halloween day.