First, you need to have unlocked these:
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 Winter you have 2 objectives for which you need to earn Influence:
> Qualify for the Winter Showcase event.
> Qualify for Horizon Spring.
Fill up the Influence bar as you have done before to first unlock the Showcase event. With that done repeat the process to unlock Horizon Spring.