It’s NOT a Mexican holiday.
What is it then?
Evidently it is a Mexican Holiday, just nothing to do with the U.S.
“Cinco de Mayo (pronounced [ˈsiŋko ðe ˈmaʝo] in Mexico, Spanish for “Fifth of May”) is a yearly celebration held on May 5 to celebrate Mexico’s victory over the Second French Empire at the Battle of Puebla in 1862, led by General Ignacio Zaragoza. The victory over the French army was a morale boost for the Mexicans.”