Sonic is celebrating the Winter Solstice tomorrow, December 21st with 1/2 Price Shakes all day long! I'm not sure that you can ever be too cold for a deal on sonic shakes. This offer is not valid in conjuction with any other special deals.
You might want to call ahead to verify that your Sonic is participating before you head out.
Don't forget to get the Bargain Blessings toolbar to print coupons and stay up to date on all the latest deals!