Cinco de Mayo is an annual holiday in the United States that occurs on May 5, with the focus being the celebration of Mexican Culture.
Originally, Cinco de Mayo was celebrated due to the Mexican victory in the Battle of Puebla on May 5, 1862. After a civil war in which Mexico suspended debt payments to European countries, Mexico was invaded by the French under Emperor Napoleon III, who used the debt payments as an excuse to conquer Mexico. A very small Mexican army was attacked by a better-equipped and more numerous French invasion army, yet delivered a decisive victory. Though France would go on to occupy Mexico until driven out in 1867 by the Mexicans with U.S. support, the battle featured a significant boost in morale that was needed to further resist the French, who had seldom been defeated since the Battle of Waterloo in 1815.
Ironically, Cinco de Mayo is not observed as a national holiday in Mexico, though Puebla and Veracruz call it such. In the United States, California originally celebrated Cinco de Mayo as a state holiday, and it is now a national celebration of the Mexican culture, and celebrated as such. Though I do not get school off today, Cinco de Mayo is still an important day on any calendar.