Explained to Ed, as per request:
Cinco de Mayo (Spanish for "fifth of May") is a holiday held on May 5. It is celebrated nationwide in the United States and regionally in Mexico, primarily in the state of Puebla, where the holiday is called El Dia de la Batalla de Puebla (English: The Day of the Battle of Puebla). The date is observed in the United States as a celebration of Mexican heritage and pride, and to commemorate the cause of freedom and democracy during the first years of the American Civil War. In the state of Puebla, the date is observed to commemorate the Mexican army's unlikely victory over French forces at the Battle of Puebla on May 5, 1862, under the leadership of General Ignacio Zaragoza Seguín. Contrary to widespread popular belief, Cinco de Mayo is not Mexico's Independence Day—the most important national patriotic holiday in Mexico.
TL;DR - Cinco de Mayo (fifth of may) is a big Mexican holiday.
Sinko de Mayo.