Assassin’s Creed Mirage releases on October 5, 2023.
The original October 12 release announcement arrived in May 2023's PlayStation Showcase in May 2023, but in August Ubisoft announced that it will actually arrive one week early, on October 5. At release, Mirage will be on PC on the Epic Games Store and the Ubisoft Store. It's likely that it'll eventually arrive on Steam as well, as Assassin's Creed games tend to, but it might be a wait. Assassin’s Creed Valhalla didn’t show up on Steam until two years after its initial release.