Bethesda have just released an email advertising that Fallout 4 will be free to play this weekend (25th-28th May) only. For anyone who has a steam account or Xbox Gold, now would be a good time to have a play around and pick the game and its downloadable content up for up to 67% off.

Play Fallout 4 For Free This Weekend

This weekend you can explore everything a post-nuclear Boston has to offer with the first-ever Fallout 4 Free Weekend. From May 25 to 28, Xbox Live Gold Members* and Steam users will be able to play all of Fallout 4's base content and try out Mods for free.

In tandem, Steam, PlayStation Store and Xbox Live will offer savings for Fallout 4's base game and Season Pass, with discounts up to 67% off.

*Offer only available from 5/25/2017 at 12:01 AM PT through 5/28/2017 at 11:59 PM PT. Xbox Live Gold subscription (sold separately) required; broadband Internet required (ISP fees apply).
— Bethesda mailing list

Update 05/26/2017: I have now seen the Steam offer and can confirm that Fallout 4 is 67% off and the Season pass has been reduced to 40%. Individual DLC packs have not been reduced and remain at full retail price. If I get confirmation of the Xbox prices I will update these as well.

Update 05/27/2017: Apologies for the comments being disabled, not sure how/why that happened, but they are now open again.