Everyone's Irish on St. Patrick's Day. Denver's largest cultural event — The Denver St. Patrick's Day Parade presented by Bellco, March 17th — has been a key event in Denver since 1962. The longest parade in the US attracting more than 200,000 plus spectators to downtown Denver each year. In fact, it's the largest St. Patrick's Day Parade this side of the Mississippi! Bring the whole family rain or shine.