Rainbow Six Siege is having a free weekend on PC and Xbox One. It's already kicked off on the Xbox One and runs until 8am on August 1st. For PC players, the weekend begins at 6pm tonight and ends at 6pm on Monday.
PC players need to install Uplay. Once that's booted up they'll see a banner offering access to the free weekend. For Xbox One you just need to head to the game's page in the Xbox Store.