Halloween

Halloween is a seasonal event in Pokémon GO that takes place in the latter half of October. It is the in-game counterpart of.

2016
This event took place from October 26th through November 1st.

Trainers received double Candy for catching, hatching, and transferring Pokémon. The required Buddy distance was reduced by a quarter. This meant that Pokémon received Candy four times faster.

Trainers saw an increased spawn rate for scary Pokémon. These Pokémon included: Zubat, Golbat, Gastly, Haunter, Gengar, Drowzee, Hypno, Cubone, and Marowak.

2017
This event takes place from October 20th through November 2nd.

New Ghost-type Pokémon from the Gen III series of Pokémon Sableye, Shuppet, Banette, Duskull, and Dusclops will become available to capture in the wild. A free Mimikyu’s Disguise Hat will also be accessible from the Wardrobe. There will also be a new costume Pikachu wearing a wizard/witch hat.

Double Candy will be awarded for catching, hatching, and transferring Pokémon; and Buddy Pokémon will find Candy twice as fast. Special boxes, featuring items such as Raid Passes and Super Incubators, will also be available from the in-game Shop.

Trivia

 * It was hinted that there will be a second Halloween event happening later in October 2017; which mentions that there may be new Pokémon available.