Half price Sonic shakes December 21 Dec 192012 Participating Sonic drive-in restaurants are offering shakes for half price. Mention the discount when ordering a frosty treat December 21, which happens to be the Winter Solstice. Thanks to Wichita on the Cheap for the info. If you're new here, you may want to subscribe to our daily email updates so you won't miss a deal. Thanks for visiting! Sorry, the comment form is closed at this time.