"Valve plans to announce Half-Life 3 (or Deckard, or both) on 18 November"

Valve uses the top banner on the front page of the Steam Store whenever they want to promote a Steam Sale, Steam Event, or new products like games or hardware. Whatever's on the front page of Steam gets a HUGE amount of exposure. They put Half-Life: Alyx on the front page banner immediately after the announcement back in 2019, as well as the Valve Index when it was announced. When there's a Steam Sale or Event, it takes up the top banner for the whole duration of those events (Summer Sale, Next Fest, etc.).

Therefore, they will not announce anything at the same time as major Steam Events are happening, as they would be cannibalizing their own sales. They'd rather reschedule stuff to ensure they're not cannibalizing themselves.

Here's a list of scheduled Steam Events in 2025, courtesy of /u/BeeBeeTheDev. Notice how there is an unusually long gap between Events from 18 November - 8 December.

This could mean that Valve plans to announce Half-Life 3 (or Deckard, or both) on 18 November, and made sure no other Steam Events were happening at that time. And in fact, they moved their Autumn Sale ahead by two months, to September, even though it usually happens in November, and in the Autumn Sale announcement, they wrote:

“Is Steam crazy? They’re two months ahead of schedule!”

Yes, we are crazy, and yes we are earlier than usual, but also that’s intentional.

So if it's happening in November, it'll be 18/19/20 November. Or never.