It's not a countdown of 12 days to Christmas (obviously), but a countdown until 2012! And during this countdown, Xbox is having a sale for some of its top XBLA games, one a day.
The countdown celebration starts with half off of Bastion (from 1200 MSP to 600 MSP). Note that this is for TODAY and for TODAY ONLY. Tomorrow there will be a whole new deal. So if you haven't played this popular XBLA title–like me–you really don't have an excuse now to not pick it up–like me.
The 12 Days of sales also includes sales from now until the end of the year for certain games, game add-ons, and avatar items. Visit Major Nelson's blog for the complete list.
And yes, I have rectified my error with not buying Bastion earlier.
