Halloween

Halloween is a seasonal event in Pokémon GO that takes place in the latter half of October to celebrate real-world Halloween.

2016 Event
2016 Halloween event took place from October through November, 2016.

Features
Trainers received double candy for catching, hatching and transfer of Pokémon. The required Buddy distance was a quarter of its regular amount, which means that Pokémon received candy four times faster.

Featured Pokémon
Following Kanto-region, Ghost and "spooky"-themed Pokémon had increased spawn rate in the wild:

2017 Event
2017 Halloween event took place from October through November, 2017.

Features
Similarly to the year-before event, double amount of candies was awarded for catching, hatching, and transfer of Pokémon. Buddy Pokémon found Candy twice as fast.

Moreover, a free Mimikyu’s Disguise Hat became accessible from the Wardrobe.

Featured Pokémon
5 select Ghost-type Pokémon from the Hoenn region were released to be caught in wild and hatched from eggs: Sableye, Shuppet, Banette, Duskull, and Dusclops.

There was also Pikachu family members wearing a witch hat, temporally available to obtain through catching in wild (Pikachu and Raichu), hatching from eggs (Pichu) and evolution from one to another.

Following Pokémon had increased spawn rate in the wild:

Pokémon Eggs hatchlings
During 2017 Halloween event there was possibility to hatch Pichu wearing witch hat from 2-km Eggs.

Shop special offer
Special boxes, featuring items such as Raid Passes and Super Incubators, were available from the in-game shop.

Trivia

 * Halloween event in 2016 was the first in-game event in the game history.
 * During the Halloween event of 2017 there were several Shiny Pokémon released:
 * On October, 2017 - Shiny Sableye began appearing on the first day of the Halloween event of 2017.
 * On October, 2017 - Shiny Duskull and its family Pokémon began to appear six days into the event.
 * On October, 2017 - Shiny Shuppet and its family Pokémon began to appear 11 days into the event.
 * During the Halloween event in 2017, there were shiny forms of Gastly family members added to game data, although they weren't released to be caught in gameplay.