El Cinco de Mayo ("The Fifth of May" in Spanish) is a national celebration in Mexico. It 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.

In 1861, in response to Mexico's refusal to pay off its debt, Britain, Spain and France sent troops to Mexico; they arrived in January of 1862. The democratically-elected new government of President Benito Juárez made agreements with the British and the Spanish, who promptly recalled their armies, but the French stayed, thus beginning the period of the French intervention in Mexico. Emperor Napoleon III wanted to secure French dominance in the former Spanish colony, including installing one of his relatives, Archduke Maximilian of Austria, as ruler of Mexico.
Confident of a quick victory, 6,500 French soldiers marched on to Mexico City to seize the capital before the Mexicans could muster a viable defense. Along their march, the French already encountered stiff resistance before Zaragoza struck out to intercept the invaders.
The battle between the French and Mexican armies occurred on May 5 when Zaragoza's ill-equipped militia of 4,500 men encountered the better armed French force. However, Zaragoza's small and nimble cavalry units were able to prevent French dragoons from taking the field and overwhelming the Mexican infantry. With the dragoons removed from the main attack, the Mexicans routed the remaining French soldiers with a combination of their tenacity, inhospitable terrain, and a stampede of cattle set off by local peasants. The invasion was stopped and crushed.
Zaragoza won the battle but lost the war. The French Emperor, upon learning of the failed invasion, immediately dispatched another force, this time numbering 30,000 soldiers. By 1864, they succeeded in defeating the Mexican army and occupying Mexico City. Archduke Maximillian became Emperor of Mexico.
Maximilian's rule was short-lived. Mexican rebels opposed to his rule resisted, seeking the aid of the United States. Once the American Civil War was over, the U.S. military began supplying Mexicans with weapons and ammunition, and by 1867, the rebels finally defeated the French and deposed their puppet Emperor. The Mexican people then reelected Juárez as president.

IN SHORT:
Cinco de Mayo is widely celebrated by Mexican-Americans and their descendants in the United States. Many cities with significant Mexican and Chicano populations throughout the US schedule special events on the 5th of May. In Los Angeles, most notably, revelers fill the streets in front of City Hall for speeches and performances by Mexican entertainers on this day.
Although the celebration of this holiday has historically been limited to Latin-American communities, particularly in the southwest, it has become increasingly popular across the country and among all ethnic groups in the last 10 years. In recent years, it has been celebrated widely and in a similar style as St. Patrick’s Day — namely a celebration and identification with Mexicans, their culture, food, and music by Anglos and other unrelated ethnic groups. However, many Americans have only a cursory understanding of the significance of this day; it is often mistakenly assumed to be Mexico’s independence day, and in practice, it is usually celebrated on a superficial level.