Via the official Pokémon GO newsletter, Niantic has confirmed that another Halloween-themed event is just around the corner. According to the newsletter, Professor Willow has noted that spooky things have been happening and may be related to Ghost-type Pokémon. The event will take place later this month after the Psychic Spectacular concludes. Full details have yet to be announced, but stay tuned for any updates.
Source: Pokémon GO newsletter