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.