If they manage to catch enough Pokémon during the Pokémon GO Fest Challenge Windows, the first Legendary Pokémon will be revealed in Grant Park.
If the Trainers in Chicago succeed in defeating the Legendary Pokémon, that Pokémon will start appearing in Raid Battles around the world, after Pokémon GO Fest.
So, folks in Chicago attending the event will get first dibs on legendaries, while players around the world will get a chance to encounter the same monsters if the requirements are met.
In the trailer, we see all sorts of mythical monsters, from Ho-oh to Lugia and Mewtwo, but it’s unclear which, specifically, will start appearing on Saturday.
Niantic says that, if successful, legendaries will start appearing for everyone starting on July 23rd, but only in legendary raids.