$.59, $.79, $.99... those were the days. I pulled a
homeless stint for about a month in the 90s and was able to survive on 2 bean burritos and a water each day.
My father explained the real meaning. It goes back to 1912 and involves the Titanic. It was carrying an important delivery of Helman's mayonaise from England to Vera Cruz.
Tragically, it never made it. The people of Mexico still observe a day of mourning to this day called "Sinko De Mayo."