Barker's Bones on Facebook. It resets every Thursday (that's tomorrow) so you'll want to check that out for your chance to save even more- and who doesn't love that?!
The trick with these one-day Old Navy sales is to try to get there early, because they do sell out of certain sizes quickly. If you get there and they don't have your size, you might consider buying whatever size they do have and going back later to exchange for the right size. Obviously, that's slightly risky but it seems like tons of people have had good luck with that.