My shop teacher told me this one:
In Mexico, there was a time when people really loved mayonnaise. They ate so much, they ran out of it!
They soon ordered some more to come in by ship. The people were elated. But some had a bad feeling. The ship was titled the Titanic.
As we all know, the Titanic sank. The Mexicans were so sad they created a holiday to mourn the loss.
It's name: Sink-o de Mayo.
(Play on Cinco de Mayo, it's Sink of the Mayonnaise.)