https://www.gamespot.com/articles/last-chance-for-pokemon-go-halloween-2017-event/1100-6454547/

Halloween may have ended, but Pokemon Go players still have a few more hours to take part in this year's Halloween event. The in-game event concludes on November 2 at 1 PM PT / 4 PM ET / 8 PM GMT, making this your last day to more easily find Ghost Pokemon and earn bonuses.

In addition to increased spawns for "spooky" Pokemon like Gastly, Houndour, Misdreavus, and Cubone, this year's Halloween event introduced a handful of Gen 3 Pokemon to the popular mobile game, including Sableye, Duskull, and Banette. According to developer Niantic, "The rest of the Pokemon first seen in the Pokemon Ruby and Pokemon Sapphire video games will gradually arrive starting as early as December." Players also have a chance to catch a special Pikachu wearing a witch's hat during the Halloween event.