http://www.pcgamer.com/another-overwatch-free-weekend-is-on-the-way/

Blizzard has good news for anyone who wants to play Overwatch for free next weekend: You can. From November 17-20 you'll be able to download the Overwatch client at no charge, and then dive in and play with the full roster of 26 heroes and 16 maps, in Quick Play, Custom Games, and the Arcade. 

You'll need Battle.net to take part, available at battle.net if you don't already have it, and as with previous weekend Overfreebies, any progress you make, including levels and loot boxes earned, will carry over if you buy it. The free weekend will be available on all platforms, but alas, cross-platform play is not available. (That's not new, though, just an FYI.)