For one hour every Wednesday evening, Raid Battles with the currently featured Legendary Pokémon will happen more frequently.
Date + Time
From 6:00 p.m. to 7:00 p.m. in your local time zone on the following dates:
- September 4, 2019: Raikou, Entei, and Suicune
- September 11, 2019: Deoxys
- September 18, 2019: Mewtwo
- September 25, 2019: To Be Announced
- There will be an increased number of five-star raids during the event hour.
Source: Official Pokémon GO blog