The 12 Days of Metal Gear was a viral marketing promotional event on the official Metal Gear Solid Facebook page, created for the leadup to the release of the Steam version of Metal Gear Solid V: Ground Zeroes. Named after the Christmas carol "The Twelve Days of Christmas", the event focuses on events the player can do in Ground Zeroes set to the tune of the lyrics of the song. It started on December 12 with the first half (12-7) being uploaded, and the remaining half being uploaded one for each day until release on December 18 (which, coincidentally, is exactly a week before Christmas Day, the first day of Christmas).
- 12 Guards a Sleeping (referencing the lyric "12 Drummers Drumming" as well as the fact that the player can knock out soldiers)
- 11 Snatchers Hiding (referencing the lyric "11 Pipers Piping" as well as the events of the Jamais Vu mission)
- 10 iDroid Waypoints Set (referencing the lyric "10 Lords a-leaping" as well as one of the functions of the iDroid)
- 9 XOF Badges Found (referencing the lyric "9 Ladies Dancing" as well as one of the sidequests in the main mission)
- 8 Hind D Helicopters (referencing the lyric "8 Maids-a-Milking" as well as the extraction method in the main mission and most side missions)
- 7 Missions Total (referencing the lyric "7 Swans-a-Swimming" as well as the total amount of missions in the game as of May 1 [the main mission, the four side missions, plus the two system-exclusive extra op missions])
- 6 Cameras Spying Raiden (referencing the lyric "6 Geese-a-Laying" as well as the possibility of being spotted by a security camera in Jamais Vu and also other missions).
- 5 Cigars Smoking (referencing the lyric "5 Golden Rings" as well as Snake smoking on the main menu before selecting a mission).
- 4 Side Ops (referencing the lyric "4 Calling Birds" as well as there being 4 Side Ops total in Ground Zeroes, not counting either of the two Extra Ops Jamais Vu or Déjà Vu).
- 3 Friends Saved (referencing the lyric "3 French Hens" as well as the gameplay element of retrieving prisoners and even enemy soldiers in some cases during both Main Ops and the Side Ops).
- 2 Extra Ops (referencing the lyric "2 Turtle Doves" as well as the fact that there are two Extra Ops in Ground Zeroes console releases post-May 1 [one Extra Op per PS3/PS4 and Xbox 360/Xbox One system pre-May 1]).
- And a Steam Discount for Metal Gear Solid V: Ground Zeroes (referencing the lyric "And a Partridge in a Pear Tree" as well as the announcement of Ground Zeroes release).