Play the game for free between 2 and 5 february. Required Uplay and an Ubisoft account.
FAQ .Preload is available now, to begin your download navigate to your Rainbow Six Siege game page. .The free weekend is activated by clicking on the R6S news banner in Uplay which will give the user an installation prompt .Rainbow Six Siege will be -50% off during the event .All progress made during the free weekend is kept .All content and maps is available to free weekend users
Edit: It will also be free on Steam. Thank you to Tatsuya for the information.