When the Titanic sailed from England, in the hold were 20 thousand cases of French Mayonaise bound for Mexico City. They were to be offloaded at a Mexican port city and then reshipped to the capital. When the ship was struck by an iceberg and sunk. The Mexican government was heart stricken at the number of people lost and of course their loss of their special cargo. So to commemorate that tradgedy they decided to name a Mexican holiday for it. Hence the name: Cinco de Mayo.
When we lived in town our neighbor (not the sharpest knife in the drawer) said, "This Cinco de Mayo thing comes around about the same time every year, doesn't it?" With a perfectly straight face, my husband said, "But not in leap year."