Don't usually post leaks but this seems like a very solid one given the GAME staff info sheet shown in this story and the fact that it's the same date as the multiplayer deep dive. So it's the 28th of August.

Don't forget that you DO NOT need to preorder to be able to take part in the BETA if you are a Gamepass subscriber.

Black Ops 6’s beta could launch in late August for pre-order customers | VGC

Activision is planning a multiplayer reveal event at Call of Duty Next on August 28…

www.videogameschronicle.com