If you've ever wondered about the origins of Cinco de Mayo, here's a short article that nicely summarizes the popularity of the holiday in the U.S.:
Cinco de Mayo celebrates the Mexican victory in the Battle of Puebla on May 5, 1862, against the invading French army of Napoleon III...
"Cinco de Mayo" is an official Library of Congress subject Heading. In other words, you can conduct a Subject search in Orbis, Yale University Library's catalog, and you will find several books on the holiday as well as the famous battle in the Mexican state of Puebla.
Have a look also at the U.S. House of Representatives, Committee on International Relations', Resolution recognizing the Mexican holiday of Cinco de Mayo.