The freebie runs on Tuesdays from opening to 7 PM.
The promotion is an annual tradition for the chain that they call "Cow Appreciation Day" (in that you show your appreciation for cows by eating chickens instead). It's been going on for 15 years with over 2,400 participating restaurants.
According to the Chick-fil-A, approximately 1.95 million people participated in last year's Cow Appreciation Day.
Examples for what passes as a cow costume or apparel include: "a spotted accessory, black and white spots on t-shirt, a cow hat or mask, etc."
Photo via Chick-fil-A.