The free weekend announcement trailer is below. At the same time, you'll be able to grab Fallout 4 and the Season Pass at discounts up to 67 percent off on Steam, PSN, and Xbox Live.
If you're playing on Xbox One, you'll need to have an active Xbox Live Gold membership to be able to take part in the free weekend. On Steam, the free play runs from 10 AM PT on Thursday, May 25th, until 1 PM PT on Sunday, May 28th. Steam discount period runs 25/5/2017 through 29/5/2017 at 10 AM PT.
On PS4 (and Xbox One) the discounted game and its add-ons will be available from May 25th at 9am to May 29th at 8pm.