On the Microsoft store's official page for the Forza Horizon 3 demo, it says the release date is September 11th.
It does also, rather pointedly, say it is available on Xbox One with no mention of a PC release. The full game is definitely going to be released on PC as well, but the demo might be restricted to consoles, at least for now.
The most important question we all have remains unanswered as well; will we be able to drive the Warthog in the demo? I suppose we'll have to wait to find out.