They're calling it an "UnTurkey Day" event and it's only good on single-patty cheeseburgers.
Sonic Cheeseburgers feature American cheese, crinkle-cut pickles, chopped onions, shredded lettuce, and sliced tomatoes on a beef patty with your choice of mustard, mayo or ketchup (you can check out my previous review of one here). They normally cost around $3.50 or so (varying with location).
If the picture is any indication, Sonic appears to have change the assembly for their cheeseburgers so that most of the components are on top of the burger rather than below.
Photo via Sonic.