152 years ago . . . In 1862! The Mexican army scores an upset victory over the invading French army at the Battle of Puebla.
A few important points about Cinco de Mayo, though:
1. Even though the Mexicans held off the French that day, they didn't win their war. About a year later, the French successfully occupied Mexico City. They didn't leave until the U.S. strongly pressured them to leave a few years later.
2. It's NOT Mexican Independence Day. That's September 16th.
3. It's NOT a federal holiday in Mexico, but just a voluntary one. And it doesn't get huge celebrations in Mexico . . . in fact, they barely notice it.
4. Even though it's virtually ignored in Mexico, outside of the country it's evolved into a day of BROWN PRIDE. Which is why it gets huge celebrations in U.S. cities with large Latino populations.