Mewtwo is coming to Pokemon Go, but its exact arrival date is still a mystery. Earlier this month, Pokemon Go announced that Mewtwo would appear in special "Exclusive Raids" that would start appearing sometime in the not so distant future. Mewtwo made a cameo appearance at the Pikachu Outbreak Festival in Japan, but this powerful Legendary Pokemon hasn't appeared since.
While we don't know the exact date that Mewtwo will appear in Pokemon Go, we can make an educated guess. Pokemon Go already announced that the game's four current Legendary Pokemon Articuno, Zapdos, Moltres, and Lugia would all depart the game on August 31st. Likely, Pokemon Go wanted to "clear the board" before Mewtwo's arrival to encourage as many players to show up to the Exclusive Raids as possible.
If Mewtwo doesn't appear until after the Legendary Birds leave the game, that means the earliest it could arrive is September 1st, which happens to be the Friday before Labor Day. Labor Day is a national holiday in the United States, which means that a lot of Pokemon Go players will have a long weekend on their hands. We wouldn't be surprised to see Exclusive Raids starting to appear around Labor Day weekend...which means that the game would need to start distributing Exclusive Raid Passes several days beforehand.
I think the best indicator of Mewtwo's arrival is a new Pokemon Go update. Although Pokemon Go already has the assets needed to make Exclusive Raids a reality, there's probably still some code that they're looking to refine before the new feature goes live. After all, they don't want players upset that they didn't get a chance to catch Mewtwo due to a buggy feature.
We're expecting to see Mewtwo appear in the next few weeks, but in the meantime, be sure to enjoy the last days of August and go out and catch some Legendary Birds!