Starting this month, Pokémon Go players will be able to participate in monthly events that will bring out special Pokémon with exclusive moves. Community Days will be worldwide events and to take part, you'll just have to head outside to your local park. Each Community Day will feature a special Pokémon that will pop up more frequently for a few hours. Other event perks could include increased XP or Stardust, and Lure Modules activated on Community Days will last for three hours.
Source: Pokémon Go