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 complete the 3 x Season events/races for every season.
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.
Make sure you set the Drivatars to highly skilled to obtain the three stars. Each event consists of three races you don't need to win the races you only need to have the most points after the three races. Keep an eye on the championship table if you were beaten by on driver in the first race make sure you beat them in the second etc.
1st gets 20 points
2nd gets 16 points
3rd gets 14 points
4th gets 12 points
5th gets 10 points
6th gets 8 points
7th gets 6 points
8th gets 5 points
9th gets 4 points
10th gets 3 points
11th gets 2 points
12th gets 1 point
You can finish the race and restart if you don't have enough points after you last race.
Repeat this on the other 3 seasons to obtain the achievement. I would advise to complete all the challenges on all 4 weeks which will get you these
The Encore? achievement in Forza Horizon 4 worth 426 pointsComplete all activities in a single Festival Playlist Series.