A post on Reddit claims that the Steam Holiday Sale will start on December 18th, and run through January 2nd. The source they say is an email from PayPal stating when the sale will start and end.
PayPal was caught releasing the news of the Steam Autumn Sale which later turned out to be rather accurate. The reason that PayPal always knows when the sale will begin? The answer is quite simple Steam allows users to purchase games with PayPal and therefore they alert them that there is going to be a rather large surge in sales and to basically be ready.
Now whether or not the dates were released on purpose is up for debate, but this becoming an all too common thing with PayPal. Not that it does us any harm, it just seems a little odd that they always release the information before Valve.
There is no information on what games are going to be discounted, but chances are some of the largest titles will be on sale this time around. The Steam Summer Sale this year discounted a lot of games including Far Cry 3 and Dayz maybe we will have another sale on Skyrim.