From October 23rd, 2018 until November 1st, 2018, Ghost-type and Dark-type Pokémon originally discovered in the Sinnoh region will be available to capture as part of Pokémon GO's Halloween celebration.

Have a haunting good time during Pokémon GO Halloween 2018!

Do you feel a change in the breeze? A chill down your spine? Pokémon GO Halloween is back for a spooky good time! Ghost-type and Dark-type Pokémon originally discovered in the Sinnoh region, such as Drifloon and Stunky, join in on the fun, which begins on October 23 at 1:00 p.m. PDT! And, because you can’t have tricks without any treats, Trainers will be able to earn double Candy for every Pokémon caught until November 1 at 1:00 p.m. PDT.

During this event, it seems Professor Willow will need your help on some limited-time Special Research. 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.

Plus, the Renegade Pokémon, Giratina, will make its first appearance in Raid Battles! This Ghost- and Dragon-type Legendary Pokémon originally discovered in the Sinnoh region has emerged from the Distortion World for an extended time, so challenge it before it leaves on November 20 at 1:00 p.m. PST!

Feeling extra creepy and kooky? New avatar items featuring Ghost-type Pokémon are now available in the Style Shop! From a Drifloon Cap to a Gengar Backpack, you can show your love for these Pokémon all year round.

Steel your nerves and get ready for some strange and scary fun times! Stay safe, and happy exploring!

Source: https://pokemongolive.com/en/post/halloween2018/