Today Niantic announced the PokemonGo Halloween event for 2016 and also released a promotional video for it too! Source here. You can check out the video below:
From October 26th to November 1st, you will be able to earn double the amount of candy every time you catch, hatch and transfer your Pokémon. Also, having a buddy Pokémon out will help you earn candy four times as fast. While out and about on your travels you will also encounter more spooky Pokémon than normal, but Niantic advise that they only want to enjoy the fun too!
While out and about you should expect to see more of the following Pokémon: