Why do Mexican Americans celebrate Cinco De Mayo?

Because 4,000 Mexican soldiers smashed the French and traitor Mexican army of 8,000 at Puebla, Mexico, 100 miles east of Mexico City on the morning of May 5, 1862. more info on http://www.vivacincodemayo.org/history.htm

ANOTHER ANSWEROn the other hand...

The original question was, "Why do AMERICANS celebrate Cinco De mayo?"

As an "American", I don't celebrate that particular event.