The leagues consist of 9 divisions, Vanilla being the lowest and Legendary being the highest. In order to climb up in the leagues, you will need to reach the Promotion zone. The competition gets fiercer and the Promotion zone gets smaller as you climb higher in the leagues, but the rewards will also get better!
A regular league cycle begins each Wednesday at midnight UTC and ends in the next week's Tuesday at 23:59 UTC. During special campaigns the league cycles can be different, but the duration is always visible in game! You can see the time left in the league button or from the top of the leaderboard. Between every cycle active players' standings in each division are updated.
All players who have completed level 15 can participate. Launching a bird and completing any Angry Birds Match level signs you up for the leagues. You also receive a participation reward right after that! In every completed level each launched bird is counted for scores. Note that birds left on the playfield are automatically launched after level goals are completed, so they count as well!
Each bird that you launch accrues score for you. However, you also need to complete the level in order to get the score. Don't worry about the birds left on the play field when you finish the level - you gain points from them as well!
Participation reward is awarded every cycle to each player who joins the leagues. In addition, top players in the leaderboards receive rewards in the end of the cycle.
UPDATE! Leagues during special campaigns
As of game version 2.5.0 (released on 6th of March 2019) the league cycle is no longer forced to same length as the special campaign. You can see the time left in the league button or from the top of the leaderboard.