Ben & Jerry's

This people must be stupid, giving away their food like that. Don't they know they'll cannibalize sales? Don't they know they'll addict their customers to discounts?

Or, maybe, they have propped up a weaker evening. Maybe butts in seats is worth it to them on this day. After all, you can't take a percentage to the bank.

Only Ben & Jerry's knows, but it does seem like they are promoting a higher ticket item (sundaes, instead of cones) which introduces customers to more menu items (and creates more reasons for going).

It also looks like they are incentivizing customers to bring friends (read: introduce them to the product).

Ben & Jerry's has done similar promotions in the past. Most notably free ice cream day, scheduled just before the weather breaks in the Spring, which serves to jump start their season and create new habits.

Maybe we can learn a little something from them.