Today is Cinco de Mayo (Fifth of May). But why do many in Mexico and the US celebrate this holiday? Per this Wikipedia entry, Cinco de Mayo celebrates a Mexican victory over French troops in the Battle of Puebla, which occurred on May 5, 1862.
Cinco de Mayo
1862 was a rough year for Mexico. Britain, Spanish and French troops entered Mexico seeking payment for growing debt Mexico was accumulating. Mexico made arrangements with Britain and Spain but the French remained. Then France attacked Mexico with French troops outnumbering Mexican troops 6500 to 4500. Under the leadership of General Ignacio Zargoza, the underdog Mexico defeated France in the aforementioned Battle of Puebla.
Dieciséis de Septiembre (Sixteenth of September)
Unfortunately, Mexico won the battle but lost the war. The French soon sent 30,000 troops to Mexico, ultimately defeating Mexico. The French then occupied Mexico City and French Archduke Maximillian became Emperor of Mexico. Mexico eventually won back their independence (in 1867) and celebrate their independence on Dieciséis de Septiembre (Sixteenth of September).
US President Abraham Lincoln was appalled by the French actions but was too busy with the US Civil War to be able to help the Mexicans during this war. But shortly thereafter, the US government began supplying Mexicans with weapons and ammunition, and the French were driven out on September 16, 1867.
Mexican Holidays and Celebrations
A Chicano movements in California in the 1960's brought the Dieciséis de Septiembre celebration to the US. However, September 16 proved too early in the school year to organize student rallies and celebrations, so student organizers began celebrating Cinco de Mayo instead. This move turns out to have been quite successful, since Cinco de Mayo is celebrated across most of the US on May 5, even though most don't know the root of the celebration.
By the way, there's no Wikipedia entry for Cinco de Mayo written en español (in Spanish), the native language of Mexico. See the proof here. But the Wikipedia is indeed provided en español as we as in almost 200 other languages (chose another language here). If you'd like more info on Wikipedia, visit our review of the Wikipedia.