Half price Sonic shakes December 21 Dec 192012 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!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. Sorry, the comment form is closed at this time.