Ubisoft announced the closed alpha dates for For Honor via the Ubiblog.

“And you might even be able to play early, if you qualify for the Closed Alpha running from September 15 through 18.”

The Closed Alpha will probably feature the same For Honor Gamescom 2016 build, where you can play three maps, six heroes and learn a little bit about the For Honor progression system.
You can sign up right now, but Ubisoft notes that a selected group of people will get into the For Honor closed alpha.