In one of the smaller features of the game, the Check In, you will often receive data from the Internet about various things going on the Internet, as well as receiving a daily 500 coin bonus. When you connect, you will get a noticeboard about various events being held and will be offered the opportunity to download these events.
Like events in the main game, these events are timed and often provide the only means to get certain Pokémon. These events are all placed in the special Event Stage part of the game.
As of February 13th 2018, these special events changed to be on a weekly cycle, allowing for you to be sure to get all the event PokĂ©mon. February 13th begins the weekly cycle.