This is found inside the festival playlist (you must start in summer and progress all the way through to spring, seasons change every Thursday) you need to get 80% completion of the season events which will award 2 cars 1 at 50% and the other at 80%.
Getting the first car will get you
The Festival playlist can be found on the left on the home page next to the world map. It is also shown when you load up the game.
The events are 1 x weekly challenge, 7 x daily challenges (each challenge is active for 3 days before changing), The Trial, Seasonal playground games, 3 x Seasonal PR stunts, 3 x Season events/races (Drivatars must be Highly skilled or higher to get three stars) , 1 x monthly Rivals and 1 x online adventure. Monthly Rivals and online adventure only need completing once in any season to count.
As you complete these events you will see the Festival playlist and Horizon season completion percentage increasing once you hit 80% in the current season and are awarded the second car you can wait until the next season. Repeat this on the other 3 seasons to obtain the 8 bonuses for the achievement. I would advise to complete all the challenges on all 4 weeks which will get you these
You also get 2 additional cars for getting 50% and 80% of the festival playlist completion but these don't count towards this achievement.
The Encore? achievement in Forza Horizon 4 worth 339 pointsComplete all activities in a single Festival Playlist Series.