Niantic has announced that during September of 2019, for one hour every Wednesday evening, Raid Battles with the currently featured Legendary Pokémon will happen more frequently.

Get ready for Legendary Raid Hours in September!

Trainers, We’re excited to announce that there will be more Legendary Raid Hours taking place in September! 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

Features

• There will be an increased number of five-star raids during the event hour.

As always, Trainers, be mindful of your surroundings and of others around you while playing Pokémon GO. Stay tuned for more updates.

Happy raiding!

Source: https://pokemongolive.com/en/post/legendaryraidhoursseptember2019/