Update
The european beta will now also launch on October 27th!
UPDATE (October 23, 2015): We were able to complete setup and testing of our beta servers for Europe sooner than anticipated. As a result, we're happy to report that we will be launching the Overwatch Closed Beta in both the Americas and Europe gameplay regions on October 27.

Original Post
After what has seemed like forever since people were able to register for Overwatch beta it looks like Blizzard is ready to allow the masses to test their (unfinished) game before it goes live in earnest. Starting as of October 27, 2015 select players will be granted access to the closed beta, with plans for larger testing to occur on special, designated, weekends.
The Overwatch beta officially begins October 27, and we're getting ready to invite our first batch of recruits for some field-testing! Read the mission briefing below to learn more about our plans for the beta, and don't forget to sign up if you're interested in helping us shape the fight for the future.

We're aiming to accomplish two primary goals with our public beta test: First, get tons of top-notch feedback on the gameplay"including balance, feel, and fun"to help us make Overwatch the best team-based shooter on the planet. Second, we want to hammer the heck out of our tech, including stress-testing our server infrastructure and making sure the game runs great on the widest variety of systems possible.

To accomplish this, we're dividing our beta participants into two different groups, each with different mission objectives and deployment schedules. The Closed Beta group will form the core of our testing crew. In addition, we'll be conducting a number of Beta Test Weekends with wider groups of players when it's time to break out the big guns.

Blizzard are hoping that these tests give them the opportunity to create a better end product by allowing the general public to provide criticism before the title is reaches version one. Take this chance, and sign-up for the beta!

 
Closed Beta
  • The Closed Beta will be composed of a small number of testers who will have regular access to our public beta test. Our goal for the Closed Beta is 100% gameplay feedback, and we'll be encouraging participants to discuss and dissect every hero, map, ability, and other aspect of the game on our upcoming beta forums.
  • In order to get the most meaningful and constructive feedback possible, the total number of players in the Closed Beta will be extremely limited"though we'll be adding more people from time to time to make sure we have the right number of heroes actively engaged.
  • The first phase of the Closed Beta will deploy on October 27, 2015 in the Americas gameplay region, and players may be added gradually to start (with more being invited as new features are ready to test). During this time, we'll continue to make preparations for our Europe and Asia gameplay regions"our goal is to have Europe come online next, with Asia following at a later date.
 
Beta Test Weekends
  • From time to time, we'll also need to open up the floodgates and call upon an army to overwhelm our hardware. The goal: get as many heroes playing as we can, all helping us "stress test" our servers to ensure they can withstand the assault.
  • These larger-scale Beta Test Weekends will be brought online occasionally for a limited time, generally over a weekend, and will run concurrently with our ongoing Closed Beta test. Because Beta Test Weekends are primarily hardware- and tech-focused, the number of heroes, maps, and gameplay modes will be restricted"but feedback on everything will definitely be welcome.
  • Multiple different Beta Test Weekends will be assembled in various parts of the world. We have tons of would-be heroes signed up for the Overwatch beta, and we want to give as many as possible a chance to play during the testing period. The first Beta Test Weekend is currently slated for after BlizzCon"stay tuned for more information following the event.
 
Beta Access Details
  • The Overwatch beta will be Windows-only.
  • You WILLll need to have the Battle.net desktop app installed to play.
  • If you're selected to participate in either the Closed Beta or one of our Beta Test Weekends, you'll receive an email with appropriate instructions.

Source: icon_ow Overwatch Blog