Many years ago on this day, in Mexico the locals were having a huge celebration about their victory over the French. They had beer by the truckload, cold cuts fresh from the butcher's shop and freshly baked bread. They had mustard and catsup delivered by the barrel and were waiting for the last of the condiments, mayonnaise, to arrive by ship. Just as the ship left the channel to come to the dock, it hit a huge rock that was not on the charts, and sank.
The people on shore were aghast but decided to make the best of a bad situation and partied on anyway.
Ergo, Cinco de Mayo.