On this page of the Forza Horizon 5 guide, you will learn when and what rewards you can receive for completing weekly challenges in The Festival Playlist mode.
Forza Horizon 5 has a mode where you can complete a variety of challenges that change every week. Each week offers unique challenges and rewards for completing them.
Both the challenges and the rewards will differ from each other. However, you will usually get new, unique cars. In addition, you will also be able to win seasonal points (which can be spent in the Forzathon Shop) as well as items to customize your cars - the theme of these rewards will change over time.
As for redeeming the rewards, you will receive them the moment you complete the given weekly challenge.