The game’s multiplayer is structured into seasons. Each season lasts for 30 days. All players may see how much time is left until the end of the season in the Ranking Menu. You can see it in the section called: “PLAYERS” in the Leaderboard.
After a season ends, all trophies are being reset and all the players with 2800+ trophies are rewarded. The higher the League, the better the reward. You may look up the rewards by tapping the “I” icon in the Arena Menu. It’ll show you the predicted reward, i.e. from a Bronze to a Diamond League.
On Season’s end, the surplus of trophies above 2800 is cut in half and added to the basic minimum. See the example:
If Adrien has 3200 Trophies at the end of a Season, he’ll get 3000. Why?
3200 - 2800 = 400
400/2 = 200
2800 + 200 = 3000
If Adrien has 10000 Trophies at the end of a Season, he’ll get 6400. Why?
10000 - 2800 = 7200
7200/2 = 3600
2800 + 3600 = 6400
All players may look up the Trophies number reduction by going to the Arenas Menu and finding the purple ribbon below the Bronze League Icon which says “LEAGUE SEASON RESET ___”.