As players increase their Ranked rating, they will qualify for higher leagues. Once Weekend Classic begins, players will be locked into their current league and cannot promote or demote leagues. The league a player qualifies for is based on the maximum rating a player has achieved in the most recent Ranked.
For example, if a player currently sits at 550 Ranked Rating when a Weekend Classic starts but had a max Rating of 650 earlier in the current Ranked, they will qualify for the Silver II League.