https://www.gamespot.com/articles/forza-horizon-4s-first-free-seasonal-events-and-ca/1100-6462061/

Forza Horizon 4 introduces a new mechanic, letting you experience the same highways and back alleys with vastly different road conditions based on the season. Seasons will cycle weekly, starting with summer when the game launches, and offer special time-sensitive events and loot. Microsoft has now detailed the first four weeks of content, which will run through the full seasonal cycle.

As detailed on Twitter, each season will host a selection of new seasonal events--five every season except winter, which will have three. All of them also have seven new daily events and one new weekly event in Forzathon, two new cars for car pass owners, and new stock in the Forzathon Shop. A new exclusive car will be available in each season too: the Ferrari 812 in summer, 911 GT3 RS '19 in autumn, Chevrolet ZR2 in winter, and Honda Civic '18 in spring.