The Halloween 2018 festivities for Pokémon Go kick off today at 1:00pm Pacific. The event is your chance to score Ghost- and Dark-type Pokémon including Sinnoh-region regulars like Drifloon and Stunky. The legendary Giratina is also making an appearance in Raid Battles until November 20.
Best of all, Niantic is flipping the switch for double candy payouts while you're out there chasing ghosts. The double candy bonus will remain active until Thursday, November 1 at 1:00pm Pacific.
"During this event, it seems Professor Willow will need your help on some limited-time Special Research," notes Niantic. "He stumbled across an odd stone one day and has been experiencing strange happenings around his lab ever since! Could a Pokémon be causing this ruckus? Complete Special Research tasks and find out before whatever it is goes away! You’ll also be able to enjoy Field Research featuring Ghost-type and Dark-type Pokémon throughout the event."
Considering the Halloween artwork features Spiritomb, I think we have our culprit. Not-so-fun fact: it's formed by 108 spirits (!) and it's "bound to a fissure in an odd keystone for misdeeds 500 years ago."