Niantic has announced that on Friday, October 23rd, 2020, at 6:00 p.m. PDT, Mega Gengar will be available to battle in Mega Raids, and Mega Venusaur will be leaving Mega Raids.

Mega Venusaur leaves Mega Raids on October 23. Plus, take a sneak peek at some Mega Evolution updates.

Trainers, On Friday, October 23, 2020, at 6:00 p.m. PDT (GMT −7), Mega Gengar will be available to battle in Mega Raids, and Mega Venusaur will be leaving Mega Raids. Until then, Mega Venusaur will be appearing more frequently in Mega Raids, so be sure to make the most of this time! Here are some other ways you’ll be able to earn Venusaur Mega Energy.

• You’ll be able to earn Venusaur Mega Energy by completing select Field Research tasks.
• An upcoming event in November will feature Timed Research that rewards Venusaur Mega Energy.

We’ll provide other ways for you to earn Venusaur Mega Energy in the future, so stay tuned for updates. In early November, there will be updates to the Mega Evolution feature. You’ll be able to do the following.

• Earn Mega Energy by walking with your Buddy Pokémon if you have previously Mega Evolved a Pokémon in your Buddy Pokémon’s Evolutionary line.
• Earn additional Candies when you have an active Mega-Evolved Pokémon and you catch a Pokémon that shares a type with your Mega-Evolved Pokémon. You will also earn additional Candies when catching a Raid Boss, regardless of its type.
• See Trainers on your friend list who currently have a Mega-Evolved Pokémon.

Good luck in your raids, Trainers! Please be aware of your surroundings and follow guidelines from local health authorities when playing Pokémon GO. Upcoming events are subject to change.

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