It's limit one donut per costumed guest, but you can get any donut of your choice that's available when you get there (including the seasonal Halloween and Ghostbusters donuts), while supplies last. No purchase is necessary.
The offer is only good on Halloween i.e. October 31, 2014 and is specifically not valid at Albany, GA, Pigeon Forge, TN, Council Bluffs, IA, Clive, IA, Omaha, NE, Panama City, FL, Dothan, AL, and Uncasville (Mohegan Sun) locations. Unlike the Chipotle deal, you can pick up your free donut all day.