Sorry, but Cinco de Mayo is not equivalent to St. Patrick's day.
St. Patrick is the Patron Saint of Ireland and his birthday, March 17, has been celebrated in Ireland for centuries. It is a big, national holiday there and has been that way for hundreds of years.
Cinco de Mayo has never been a national holiday in Mexico. Most Mexicans are perplexed by the way it has been made into a big deal here. It's an obscure regional celebration in Mexico and a manufacured holiday here.