Benito Juarez was the President of Mexico who resisted the French Occupation of Mexico, overthrew the Second Mexican Empire and restored the Constitutional Republic.
The reason people celebrate Cinco de Mayo is the unlikely victory of Mexican troops over French forces in the battle of Puebla. Many people north of the border confuse Cinco de Mayo with Mexican Independence Day, which is actually commemorated on September 16th.
For many Americans Cinco de Mayo is an excuse to party. Nationally, it is now the third biggest bar night of the year. But considering the way we celebrate and the general ignorance about El Día de la Batalla de Puebla, maybe it's better to consider it the Revenge of the Fifth.