Though everyone thinks it is. Instead:

Cinco de Mayo ("Fifth of May" in Spanish) is a national holiday in Mexico which commemorates the victory of Mexican forces led by General Ignacio Zaragoza over the French expeditionary forces in the Battle of Puebla on May 5, 1862.

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Why not Mexican Independence day?

The rise in Cinco de Mayo's popularity in the United States can be attributed to the Chicano student movement of the late 1960's. Inspired by student-activists nationwide, members of the MEChA organization in California sought to find a day of celebration that highlighted their largely Mexican ancestry. “El Dieciséis de septiembre” (September 16) seemed like an obvious choice, however this day proved too early in the school-year for college students to effectively organize rallies and celebrations. Thus Cinco de Mayo became the de facto alternative for these student assemblies.

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Of course Corona's promotion of the holiday has certainly helped all Americans embrace it. This year, however, is a especial Cinco de Mayo since it's Cinco de Mayo de Cinco or 05/05/05.