https://www.gamespot.com/articles/pokemon-go-announces-may-2018-community-day-detail/1100-6458281/

Pokemon Go's fourth Community Day may now be behind us, but players can already begin looking forward to next month's event. Niantic has announced the first details for May's Community Day, which is scheduled to take place on Saturday, May 19, and will feature another very popular Pokemon from the Red and Blue versions.

The featured Pokemon during May's Community Day is another Kanto starter: the Fire-type Charmander. During the event, Charmander will spawn in much greater numbers than usual. Niantic hasn't revealed what special move the Pokemon will be able to learn, though if the past few Community Days have been any indication, players will presumably need to evolve Charmander into Charizard before the event ends to learn the exclusive attack.