7/11 (July 11th) is free slurpee day at 7-Eleven each year. Don't forget to stop in and enjoy this free opportunity.
I have not participated in this event in the past (but will this year), but it sounds like they have special sized cups you use that are 7.11 ounces. You walk in get your free cups, load up and be sure to tell the clerk thank you. Event ends at 9pm in most stores, and it should be honored nationwide but never hurts to call ahead and make sure your store is participating.
I got this from Centible in Seattle.