Battle Heatran in raids from August 21, 2020 at 1:00 p.m. PDT to September 10, 2020 at 1:00 p.m. PDT (GMT −7)
Be sure to assemble a team that includes Water-, Fighting-, and Ground-type Pokémon to take down this Legendary Pokémon before it overpowers your team with its massive strength!
Don’t miss this chance to catch this Legendary Pokémon before it returns to its mountain home! Stay safe, and happy exploring!
—The Pokémon GO team
Source: Official Pokémon GO blog