First, you need to have unlocked this:
The first few hours of Forza Horizon 4
see you experience all 4 Seasons before you join the full "shared world".
As you complete races, challenges and other activities you earn Influence. This Influence is used to fill a progress bar which you see on your Map from the main menu, or as a pop-up while you're driving.
While in Horizon Autumn you have 2 objectives for which you need to earn Influence:
> Qualify for the Autumn Showcase event.
> Qualify for Horizon Winter.
Fill up the Influence bar as you have done before to first unlock the Showcase event. With that done repeat the process and head back to the Festival Hub when prompted and you will unlock Horizon Winter.