From August 31st until September 30th, Raikou can be caught in the Americas, Entei in Europe and Africa, and Suicune in Asian countries. Afterwards, the Pokemon will rotate locations until the end of October.
Also starting September 6th, the Pokemon GO developers will begin inviting players to try out their new “EX Battle Raid” feature before rolling it out globally. The feature allows players to battle rare Pokemon like Mewtwo.
EX Raid Battles are similar to existing Raid Battles, with a few notable differences. EX raids will periodically appear at Gyms around the world; however, unlike existing raids, Trainers will be invited to join an EX Raid Battle. To receive an invitation to participate in an EX Raid Battle, Trainers must have successfully completed a raid recently, by defeating the Raid Boss, at the Gym where the EX Raid Battle will be taking place. The invitations will include advance warning of when the EX Raid will take place, giving them ample time to coordinate with other Trainers before taking on the powerful Raid Boss.