No, it's not Mexico's Independence Day!!!! That is celebrated in September.
Cinco de Mayo is actually celebrated more widely in the United States of America than Mexico. And do you know why!?
Basically it's because Mexico outpowered the French in the 1800's, while the U.S. Civil War was going on. The French tried to take over Mexico. Mexico won. But a bit later, the French took over Mexico City. But, not without a fight....
Let me school ya a bit more via Wikipedia.
According to Mr. Donald W. Miles:
At the time, there were fears in the United States that the French would use Mexico as a base to back the Confederacy, so President Lincoln...
...and his Secretary of State went out of their way to appear 'neutral' in the Mexican situation. They did not want to take on the French and the Confederates at the same time. The French, via, Napoleon III, had hesitated to take on the United States directly, but the news of the Civil War changed everything. It meant that the Americans would be occupied with their conflict between North and South for some time. Napoleon decided he would not only send the French navy, but would also start looking for someone to place as emperor in Mexico City. He would then use Mexico as a base to help the Confederates win their war against the United States. Napoleon saw this as an opportunity not to be missed. The Emperor of France ordered his generals to spend a few months taking on Mexico and then - using Mexico as a 'base' - help the Confederates win their war against the United States.
Can you even imagine if they had succeeded? The United States would never become the significant world power it is today...the Mexicans not only took their nation back, but influenced the outcome of the U.S. Civil War.
And there you have it! Celebrate Cinco de Mayo!!!!