Pokémon Go players around the world are invited to band together and catch Pokémon during the Pokémon Go Fest in Chicago this month, unlocking rewards and bonuses for everyone.
During the Pokémon Go Fest in Chicago July 22, players who catch specific Pokémon while at the festival will grant bonuses to players around the world, including XP bonuses and encounter rate bonuses, while players around the world can catch Pokémon to increase bonus times and unlock a mystery challenge.
The mystery challenge will only be available to players at the Pokémon Go Fest, but attending players who pass the challenge will unlock a "major reward" for players around the globe.
Developer Niantic released this handy infographic to keep track of when these challenges are happening during the Pokémon Go Fest.

Additionally, Niantic announced Safari Zone events across Europe throughout August and September, with special Pokémon and raid bosses appearing in specific locations. Here is where and when the Safari Zone events will be:
Aug. 5:
Fisketorvet in Copenhagen, Denmark
Centrum Cerny Most in Prague, Czech Republic
Aug. 12:
Mall of Scandinavia in Stockholm, Sweden
Stadshart Amstelveen in Amstelveen, The Netherlands
Sept. 16:
CentrO in Oberhausen, Germany
Les Quatre Temps in Paris, France
La Maquinista in Barcelona, Spain