The deal begins at 8 a.m. ET Saturday online. If you'd rather pick it up in person, just get to your local Microsoft Store as early as you can tomorrow.
The discounted games are available while supplies last, so I'd make sure you're at the door, or online the second the sale starts.
The 12 days of deals promotions start at 8 a.m. ET every day through Dec. 19. You can find out the day's deal and the next day's deal each day, right here.