Check it out here.
Another season, another Steam sale. For the duration of December 19, 2013 - January 3, 2014, most games available are cheaper. Don't forget flash sales and daily sales which change every 8 and 24 hours respectively. And if you're a trading card fanatic, voting at the flash sales every three times gives you a trading card (which you can sell or use with others to create badges) unique for the event.
As with Autumn Sale, all Call of Duty titles with the exception of Call of Duty: Ghosts are 25% off for the duration of the sale, and there are likely additional discounts at some point. Those discounts will also be updated here.
Addendum: Call of Duty: Black Ops II and Call of Duty: Black Ops are 50% off until December 29, 18:00 (UTC).