https://www.gamespot.com/articles/pokemon-go-august-2018-community-day-detailed/1100-6460396/

With July's Community Day now behind us, Pokemon Go developer Niantic has shared the first details about next month's event. August's Community Day will kick off on Saturday, August 11, but unlike previous events, it will return for an encore the follow day, August 12, giving players two chances to participate next month.

Despite being available for an extra day, August's Community Days will still each only run for three hours, as was the case with previous events. During the designated times, players will find increased spawns of a particular Pokemon and earn bonus rewards for playing.