Fallout has turndes 20 years old, as the post-apocalyptic RPG was first released on September 30, 1997. To celebrate, the game is available for free on Steam for a limited time, an offer that apparently does not extend to other digital storefronts.
No other games in the series have received a discount to honor the game's birthday, and Bethesda has not timed any other new game or content releases to honor the date. December 12 is one: Fallout 4 VR, which will launch exclusively for the HTC Vive headset on that date for $59.99. (That price will likely only include that game's core content, as opposed to Fallout 4's "game of the year" edition that includes its extra DLC for free.)