Sonic typically sells their corn dogs for around $1 (varying with location).
It seems a somewhat arbitrary (and possibly made-up) day as school in the U.S. typically ends anywhere from late May to late June (varying from district to district).
The 50-cent corn dog promotion is a popular promotion at Sonic and is offered several times a year. There's no limit on the amount of corn dogs you can buy.
Photo via Sonic.