Cinco de mayo commemorates the May 5, 1862 Battle of Puebla (Batalla de Puebla) in which Mexican troops defeated Napoleon’s French forces. Contrary to popular belief in the U.S. , it is not the Mexican equivalent to our Fourth of July (Independence Day.) Cinco de mayo is celebrated more in the United States than in Mexico.
If you want to celebrate with your host kids, you can find some activities and info about Mexico here.