Microsoft have announced the games that will be available for free in December for Xbox Live gold members. On Xbox One The Incredible Adventures of Van Helsing (the first of three parts, already out on Steam - now on console for the first time) will be available for the duration of the month. The stealth reboot Thief can be downloaded for free from December 16th and until 15th January.
On Xbox 360 Microsoft offers three games next month. Castlestorm is free in the first half of December. Sacred 3 and Operation Flashpoint: Dragon Rising are available during the second half. All three games are supposedly playable on Xbox One with backwards compatibility. So in total, Xbox One owners will get five free games to play in December.