Xbox One owners planning to take advantage of Bethesda's free Fallout 3 promotion when you purchase Fallout 4 will have to do so before February 2016.
According to the listing on the Xbox Store, the special promotion to get a free backwards-compatible Xbox 360 version of Fallout 3 when you buy Fallout 4 on Xbox One will expire on February 8, 2016.
While the free game is for Xbox 360, we recently learned that Fallout 3 will be among the 100-plus Xbox 360 games made playable on Xbox One through the system's backwards compatibility feature, which arrives as part of the New Xbox One Experience on November 12th.