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 a quarter of its regular amount. 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 took 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.