In celebration of getting away with not paying your bills.

In 1861, Mexican President Benito Juárez stopped making interest payments to its creditors (France, England and Spain). England and Spain cut a deal with Mexico but France attacked Mexico to force payment of their debt. France also had hopes of taking over and occupying Mexico (imperialism was still in vogue).

Cinco de Mayo is a celebration of Mexico's unlikely defeat of the French at a time when the French army was considered the finest in the world.  Mexico beat the larger French forces in the Battle of Puebla on May 5, 1862.  Some will argue that this was a David and Goliath story (and it is) but I prefer to characterize it as a celebration of Mexico not paying its debts.  And getting away with it (for awhile).

The Mexican victory didn't hold up for long. A year later, thirty thousand French troops defeated the Mexican army, captured Mexico City, and installed Emperor Maximilian I as ruler of Mexico. However, the French victory was also short-lived.  After the end of the U.S. Civil War in 1866, Napoleon III, facing tough Mexican guerrilla resistance, the threat of war with Prussia, and concerned about a conflict with the United States, started retreating from Mexico. On June 5, 1867, Benito Juarez finally entered Mexico City where he installed a home-grown Mexican government.

But alas, the holiday is not celebrated as "Cinco de Junio".