It happened on May 5th. It is a big celebration within the Hispanic or Latino communities. When I lived in the Denver area, the city closed down streets due to the traffic from the celebration but what are they celebrating? Do you know?
I didn't know for the longest time because it was something that happened every year and since I lived right next to the local Hispanic community, I would go over to the park and join in the celebrations. But! I didn't know what they were celebrating.
Fast forward to when I took Spanish classes at the local community college. That is when I discovered the mystery behind Cinco De Mayo. It turns out it is not celebrating Mexican Independence which is what many people think. It celebrates a battle that Mexico won against the French.
At one point, the French occupied Mexico. Due to a series of wars, the Mexican treasury was almost bankrupt by 1861 so the president suspended all payments to foreign countries. Unfortunately, Spain, England and France didn't like it so they sent their naval forces to request payment. Spain and England reached agreements with Mexico but France decided this was a good political opportunity.
France wanted to establish a Mexican empire that would favor French interests. The French fleet stormed Veracruz causing the president and his people to with draw. The French followed them inland towards Mexico City but as they followed they ran into resistance at two forts and a town.
The poorly trained and equipped Mexican army of approximately 2000 soldiers crushed the better trained and equipped French army of nearly 6000. The French military was considered one of the best armies at this point in time, so the victory was sweet. This victory bolstered Mexican resistance but it did not prevent the French from sending additional troops who were able to overwhelm the Mexican troops and installing Emperor Maximilian I as ruler of Mexico.
This government only lasted 3 years because once the American Civil War ended in 1865, the American government was able to provide assistance to the Mexicans to rid their country of the French. According to various sources, Cinco de Mayo has been celebrated in California continuously since 1860 or so but it came to the forefront in the 1940's with the Chicano Movement.
Now, Cinco de Mayo is celebrated in the United States where there are all sorts of parades, music, foods, etc while in Mexico it is only a school holiday.