Starfield is now available to preload on Steam, and it's a hefty 116GB download. If you've preordered ahead of the RPG's semi-official September 1 release, and the option to preload isn't showing, just reboot the Steam app.
Preloading has been available on PC Game Pass for a while, so if you're a subscriber and playing it there, you're likewise able to install it right now. It's a massive download, so I recommend you do.
The global Starfield release times vary depending on how much you pay for Bethesda's new outing. If you buy the Premium Edition you gain access as early as August 31; if you're slumming it with the standard edition, or playing it on PC Game Pass, it'll launch as early as September 5.
I'm getting the sense that Microsoft wants to make a bit of cash with Starfield. That Premium Edition incentive is going to cause a lot of FOMO and a lot of impulse purchases over the coming days. Not only that, but the $1 PC Game Pass trial has rather conveniently disappeared in the days ahead of Starfield's release. I guess it's fair enough: Microsoft's first-party offerings haven't been that great for years, and Starfield needs to count.