Half Price Shakes at Sonic (12/21 Only) 12/20/2012 By Admin2 2 Comments This post may contain affiliate links, please see my disclosure policy. Need a reason for a yummy treat? Tomorrow, December 21st, head on over to Sonic for half price shakes all day! All you need to do is mention the deal when you order, no need to print a coupon or anything. You can find a sonic near you here.