Everyone's Irish on St. Patrick's Day. Denver's largest cultural event — The Denver St. Patrick's Day Parade presented by Bellco, March 16th — has been a key event in Denver since 1962. The longest parade in the U.S. attracting more than 450,000 spectators to downtown Denver. In fact, it's the largest St. Patrick's Day Parade this side of the Mississippi! Bring the whole family rain or shine.