On the day, you can get a free breakfast, lunch, or dinner combo meal (entree, side, and beverage) for dressing head-to-toe in cow attire (or as the chain likes to say, "head to hoof"). If you're not ready to go whole cow, you can still net yourself a free entree of your choosing by wearing a cow-spotted accessory, including a hat, scarf, tie, or purse.
If you need help in the dressing up department, Chick-fil-A has a handy DIY printable cow costume on their website here.
Over 900,000 customers dressed up as cows and visited Chick-fil-A restaurants last year and they're expecting to attract over 1 million customers this year (that's a lot of free meals/entrees!).
If you're wondering, "Why cows?" Chick-fil-A has incorporated the "Eat Mor Chikin" Cows in their advertisements since 1995. In an act of self-preservation, the cows exhort people to "Eat Mor Chikin" instead of beef.