While Nintendo has yet to give a concrete date for when The Champions’ Ballad DLC for Zelda: Breath of the Wild will be launching, the European eShop seems to have given us a window.
The European Switch eShop contains a listing for the DLC, that clearly says that The Champions’ Ballad is planned for December.
The Legend of Zelda: Breath of the Wild’s new DLC pack features a new dungeon, an original story, and more new challenges. Hopefully we get an official announcement soon but this sounds like very reliable information.