Many people think that it's Mexico's Independence Day (like our 4th of July); but, it is not.
(Note: Mexico's Independence day is September 16).
What Cinco de Mayo really celebrates is the Mexican victory over the French armies in the area of Mexico called Puebla (in the southern section, just East of Mexico City) in 1862.
Cinco de Mayo was so important to the Mexicans, that even Mexicans living in the United States celebrated. As early as 1863 (one year later), Mexican residents in San Francisco began celebrating "la gloriosa fecha" (the glorious date) and have been ever since representing Mexican nationalism in a foreign land. The American flag is flown along side the Mexican flag too.
Many today think of Cinco de Mayo as just a big drinking party. But, it really represents standing against strong, invading forces, oppression of a culture and of personal freedom in the lives of a country's citizens.
And now you know!