Denny's free birthday offer is a free Grand Slam breakfast. The offer is only valid on the day of your birthday, as opposed to the entire week like some restaurants, so no dilly dallying! You have permission to dilly though. There's always time to dilly.
As with almost every other free birthday food offer, you must join Denny's free rewards program ahead of time in order to get this free meal! You simply need to create an account, add your birthday, and then check your email as your birthday gets near. Denny's will email you a coupon for your free breakfast during your birthday month.