Announced via the game's official website, the open beta will run from August 12 - August 16. You'll be able to "play and experience intense four-player co‑op, competitive multiplayer as human or Ridden, and frenetic gameplay that keeps you in the action", as stated on the site.
However, if you pre-order the game, you are guaranteed with Early Access, which runs from August 5 - 9, so you'll get to give it a try one week before others.
For more details, you can check the FAQ right here.
Check the beta launch time of your region in the map below.