Rainbow Six Siege will be free to try for an entire week starting from August 28 and ending September 9. During that period, you'll have access to 20 legacy operators, and all progress won during the period will carry over if you choose to purchase the game. According to Ubisoft, it'll be a 70 percent discount off the Deluxe, Gold and Ultimate versions.
The free weekend will likely finish up just before the next expansion, Operation Ember Rise, hits some time in September. That's live on test servers at the moment, but Morgan has a good rundown of its new operators and the new Kanal map rework (which looks sweet).
The game is also getting with the times and introducing Battle Passes some time during the Operation Ember Rise season. Cheerfully, the first one will be free.