Update: It seems that we'll have to wait a bit longer for the demo. When a Twitter user asked the official Forza Motorsport account whether or not we'd get a demo at 1am BST / 2am CEST they replied with a vague:
"We are getting closer to the release of #ForzaHorizon3 and will be sharing more details on demo plans very soon. Stay tuned!"
We all expected Forza Horizon 3 to get a demo, and now we know when.
Playground hasn't said anything official, but the demo suddenly appeared on the Microsoft Store. The Store page claims that the demo will launch on Xbox One at 1am BST / 2am CEST this Monday, and weigh in at 18.34 GB.
We don't know what we'll get to try out it the demo, but it mentions multiplayer so we might get to test it against other players.