In the United States, apparently more even than Mexico, Cinco de Mayo has become a celebration of Chicano solidarity and ethnic pride. It has grown beyond the historic significance of its Mexican homeland in the same way that St. Patrick's Day has grown different from its original roots.
Cinco de Mayo does NOT mark Mexican independence. It marks an important battle at Puebla. The Mexicans defeated the French attempt to occupy their country. See these three interesting links for more history:
Cinco de Mayo at UCLA link
Interesting history and some notes about how the U.S. celebrations differ from the Mexican traditions. Several photographs. Notes on how commercial U.S. celebrations have become.
Children's Cinco de Mayo Link
Much more in-depth. Though it has an annoyingly child-URL, it is an excellent site with lots of detail and legal information with dates.
Cinco de Mayo from San Marcos, Texas Link
Interesting history with a different slant because it notes U.S. involvement. This site is about an annual menudo cookoff in San Marcos, the weekend closest to Cinco de Mayo. They describe menudo thus:
a wonderfully aromatic soup made of tripe, hominy and chili, and is stewed for hours with garlic and other spices. the broth is rich, red, papery, and glistens with fat. It stimulates the senses, arms the insides, and clears the head.
Menudo is served in big open bowls brought to the table steaming and fiery. It is usually eaten in the wee hours after a night out on the town and widely proclaimed to be an antidote for hangovers
Has lots of recipes, photos of past events. Site loads a bit slow.