I first learned about Cinco de Mayo from a college professor during an Oceanography lab class at Pierce College, a community college in Woodland Hills, the hottest part of the San Fernando Valley (which is often the hottest part of Los Angeles County, based on little more than a belly full of tequila). He explained to our whole class that Cinco de Mayo was a day of remembrance for the Mexicans, to a time when their people stood waiting on the shores of their country for a shipment of mayonnaise, a delicacy described to them in great detail by some shiny toothed European. The shiny toothed European promised to send them barrels of the stuff, and he followed through, but unfortunately the ship carrying all that mayonnaise ran into the horse latitudes where a stray horseshoe struck the hull and brought the ship to it's ocean bottom knees (on second thought, it isn't the tequila in my belly, it's the alcohol that seeped into my bloodstream...). In doing so, all that mayonnaise sunk with it, thus: Sinko de Mayo. A holiday that Mexicans celebrate to this day in honor of a sunken condiment (<-- a word that took me 30 seconds to remember due to all the ethanol in my brain). (you like that shit? tequila --> alcohol --> ethanol, PLAYAH!!)
Shut it, parentheses.
We know that Cinco de Mayo has nothing to do with Mayonnaise, it was just a funny tale that the old man told, though not as funny as the story he told of the time he was at the beach and a mother was screaming at her kid to stop swimming further away. As an oceanography teacher full of strange facts, he went over and explained to her that the angle of our California shores caused a current to carry her swimming son away, meaning that it wasn't his young fault that he was swimming further from her, but the old fault of a faulty old land full of sound towers that weren't faulty at all, but then you'd have to be a fan of good comedy to get that joke, Lady. The woman screeched at the oceanography teacher, telling him to buzz off and mind his own (let's stop here, and move on with the entry).
Cinco de Mayo commemorates the Mexicans declaration of "This land is our land" back in 1862 to the French soldiers who were trying to take it from them. Napoleon the 3rd sent his chocolate section out (kissed most dramatically by it's strawberry section, hubba hubba!) to Mexico to collect on a debt that the Mexicans decided they weren't going to honor. The French decided that they would invade, and invade they did, 8,000 of them striding around striking chihuahuas with buttered baguettes. The Mexicans ran abuzz, waking each other from siestas and interrupting fiestas to form an army of 4,000, who said "fuck this shit!" in Spanish and charged. They used strategy, don't get me wrong, and they fucked up some French shit.
The holiday, interestingly, is mostly celebrated in America. A big part of this, I suspect, is because the French came back a year later and took over Mexico City. The French ruled Mexico completely from 1863 through 1867 when finally America, dusting debris off of its civil war fatigued soldiers, asked them to take off.
I would speculate further, but that's what wikipedia is for, and I have another shot of sinko de mayo waiting for me in the kitchen (1/8 part mayonnaise sunk into a shot of tequila, drunk with the nose plugged and the toilet bowl very close).