Share the Coffee Love at Dunkin’ Donuts! September 29th is National Coffee Day, and to celebrate, you can get a free medium hot coffee when you buy one that’s medium or larger. That’s one coffee for you, and one for someone special, so you can cheers together to National Coffee Day. It’s the perfect way to share and celebrate that signature smooth Dunkin’ coffee you love. On September 29, get a free medium hot coffee when you buy one that’s medium or larger. Only at Dunkin’ on National Coffee Day!