Cinco de Mayo is a regional holiday in Mexico, primarily celebrated in the state of Puebla, with some limited recognition in other parts of Mexico. The holiday commemorates the Mexican army's unlikely victory over French forces at the Battle of Puebla on May 5, 1862, under the leadership of Mexican General Ignacio Zaragoza Seguin.
This is copied from the Wikipedia website. It was the first site found when I searched for my birthday, May 5th. There might be another cool thing that happened on May 5th, but Wikipedia doesn't know about it, therefore it doesn't exist.