https://www.pcgamer.com/play-rainbow-six-siege-for-free-this-weekend/

The Italians are coming to Rainbow Six Siege, and so is a free weekend—this coming weekend, in fact. Ubisoft announced today that interested but uncommitted gamers will be given full access to all maps, modes, and Operators at 10 am PT/1 pm ET on May 17. 

Preloading is slated to begin sometime today, so you can jump in and shoot at people as soon as the free-to-play period starts, rather than having to wait for a download to finish first. Any progress you make during the free period will carry over to the full game, should you opt to purchase it.