On Saturday, July 22, the first ever real-world Pokémon GO Fest will take place at Grant Park in Chicago, USA. During the day, there will be three Challenge Windows in which Trainers everywhere will work alongside those in Chicago to unlock global rewards. You can learn the details about how to participate via this infographic.
If Trainers around the world 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 there succeed in defeating the Legendary Pokémon, that Pokémon will start appearing in Raid Battles around the world, starting on July 23, 2017. Tune in to see the event unfold on Twitch and YouTube. Be sure to share your Pokémon GO Fest experience on social media with us. Use hashtag: #PokemonGOFest.
Source: Official Pokémon GO Team