'The sale will offer 17 days of great deals on hundreds of games, movies & TV shows and apps, including daily and weekly offers counting down to the New Year and continuing through Jan. 7.'

Deals are for all members although Gold members get more off.
Will do my best to update this post with the daily deals so there aren't loads of different posts (I also have nothing better to do).

***COUNTDOWN SALE***

Blood of the Werewolf // Arcade // 67% / 57%

Bloodrayne Betrayal // Arcade // 67% / 57%

Call of Duty: Black Ops // Games On Demand // 75% / 65%
Call of Duty: Black Ops II // Games On Demand // 67% / 57%
Call of Duty: Black Ops III – Bundle // Games On Demand // 40% / 30%

Chivalry // Games On Demand // 67% / 57%

Destiny: The Taken King // Add-On // 25% / 15%
Destiny: The Taken King – Digital Collector’s Edition // Games On Demand // 35% / 25%

Destiny: The Taken King – Legendary Edition // Games On Demand // 33% / 23%

Dishonored Games On Demand // 50% / 40%
Dishonored: Dunwall City Trials // Add-On // 50% / 40%
Dishonored: Knife of Dunwall // Add-On // 50% / 40%
Dishonored: The Brigmore Witches // Add-On // 50% / 40%
Dishonored: Void Walker’s Arsenal // Add-On // 50% / 40%

Dragon Age: Origins // Games On Demand // 65% / 55%

Dragon Ball Xenoverse // Games On Demand // 50% / 40%

Fable II (Back Compat) // Games On Demand // 75% / 65%
Fable III (Back Compat) //Games On Demand // 75% / 65%

Forza Horizo //n Games On Demand // 50% / 40%

Forza Horizon Rally Expansion Pack // Add-On // 50% / 40%

Forza Horizon VIP Membership and Cars Pack // Add-On // 50% / 40%

Frozen Free Fall: Multiplayer Character Pack // Add-On // 33% / 23%

Grand Theft Auto V // Games On Demand // 58% / 48%

King’s Quest: The Complete Collection // Games On Demand // 33% / 23%

Lara Croft Tomb Raider Anniversary // Games On Demand // 80% / 70%

LEGO Harry Potter: Years 1-4 // Games On Demand // 75% / 65%
LEGO Harry Potter: Years 5-7 // Games On Demand // 75% / 65%
LEGO Star Wars III: The Clone Wars // Games On Demand // 75% / 65%
LEGO Star Wars: The Complete Saga (Back Compat) // Games On Demand // 75% / 65%

Madden NFL 16 // Games On Demand // 60% / 50%

NBA 2K16 // Games On Demand // 25% / 15%

Need for Speed Rivals Games On Demand // 60% / 50%

Portal: Still Alive (Back Compat) // Games On Demand // 67% / 57%

Rise of the Tomb Raider // Games On Demand // 33% / 23%

Saints Row IV // Games On Demand // 75% / 65%

Soulcalibur II HD // Games On Demand // 75% / 65%

Star Wars: The Force Unleashed // Games On Demand // 75% / 65%

Star Wars: The Force Unleashed II // Games On Demand // 75% / 65%

State of Decay // Games On Demand // 60% / 50%

The Book of Unwritten Tales 2 // Games On Demand // 50% / 40%

Thief // Games On Demand // 70% / 60%

Tomb Raider Legend // Games On Demand // 80% / 70%
Tomb Raider Underworld // Games On Demand // 80% / 70%
Transformers: Devastation // Games On Demand // 50% / 40%

***DAILY DEALS***
23RD DECEMBER:
.skate // Xbox 360 Game // 80% / 70%
Skate 2 // Xbox 360 Game // 75% / 65%
Skate 3 // Xbox 360 Game // 65% / 55%

22ND DECEMBER:
Need for Speed Hot Pursuit // Xbox 360 // 80% (£5.99) / 70% (£3.99)

http://majornelson.com/2015/12/21/the-official-list-of-deals-on-xbox-stores-countdown-the-biggest-sale-ever/

24TH DECEMBER

Terraria // £4.20 Non Gold / £2.99 Gold
- KongDonkey

28th December
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LEGO Marvel Super Heroes - £6.25 for Gold members / £8.75 for non-Gold
- KongDonkey