The first St. Patrick's Day parade wasn't held in Ireland but in the U.S. Well, technically "the colonies." In 1762, Irish soldiers serving in the English army celebrated the holiday by marching through the streets of New York City. By 1848, the parade was an official city event and today nearly 3 million people line New York City's streets to watch the five-hour-long, 150,000-participant procession.
Chicago invented its own St. Patrick's Day tradition: it dyes the Chicago River green. In 1962 sanitation workers realized that the green vegetable dye they used to check for illegally dumped sewage could double as a St. Patrick's Day decoration. The city has been greenifying its waterways ever since. Unfortunately, the color lasts only for a few hours.