My cats turned 7 years old yesterday. Or rather, we celebrated them turning 7 yesterday. We don't know their exact birthday, having gotten them from a pet store rather than a private sale, but we know they were born sometime in early March, 2003. So we decided, for fun, to designate March 3rd their birthday, so that we could say they were born on 03/03/03.
They celebrated by playing with catnip toys, getting lots of pets and cuddles, and then sleeping a lot. I'm pretty sure they enjoyed their day, and so did we!