The Steam Winter Sale was leaked twice by Paypal as coming in November and December 18. Paypal recently outed it as December 22. Three time's the charm, as the Steam Winter Sale has officially begun, and will be running from now through to January 4.
Valve is also offering three Steam trading cards per day for subscribers who set up a Discovery Queue, a customized Store page that's tailored to the player's selections. The queue changes day to day, and players can customize it if they want.
Valve found that the Queue was highly profitable for them; in a report back in March, the company noted that game views went up a whopping 75% through the Queue during the holiday sale.
Unsurprisingly, the Queue has been brought to its knees by the massive load of the Christmas rush on Steam, so if you see "The Steam Store is experiencing some heavy load right now. Please try again later," don't be surprised.