Cinco de Mayo is Spanish for the fifth of May.
The Mexican army won the battle despite being smaller and ill-equipped.
Cinco de Mayo is not Mexico's independence day from Spain, which occurred in 1810 and is celebrated on September 16.
In Mexico, Cinco de Mayo is observed with political speeches and battle reenactments.
In the 1960s and 1970s, the day became identified with the Chicano rights movement in the United States, especially in the state of California.
The day became one to celebrate Hispanic heritage.
In many cities in the United States, there are parades and festivities featuring mariachi music, dancing and Mexican food.
Cinco de Mayo has been criticized for becoming too commercialized in recent years.