Fallout 4 will be free to play on both PC and Xbox One this weekend. However, only Xbox Gold members are able to participate in the event. The event happens at different times for PC and Xbox, Xbox players will be able to play the game starting right now until January 28th. PC players will have to wait until February 1st, the PC event will then end on the 4th.
The base game, GOTY edition, and all of the DLC are on sale during these times as well. The full breakdown of the sales can be found using the source link below.