When a guild queues for war, the game matches the guild against other guilds with the same season rank. If there are multiple guilds queued with the same season rank, the game tries to find a guild with a similar MMR. Your guild’s MMR goes up and down every time your guild wins or loses a war. Your final MMR from the previous season carries over to the next season.

Matchmaking happens on odd days at 11 AM CT.