Happy St. Patrick's Day!
According to the book Holidays, Festivals, and Celebrations of the World Dictionary the association of St. Patrick with the shamrock stems from his supposed use of its three-part leaf to explain the concept of the Holy Trinity. In the same book it states that the St. Patrick's Day Parade in New York City, which dates back to 1762, is the largest in the United States. More than 125,000 marchers participate in this parade. Even older than New York's parade is Boston's St. Patrick's Day Parade, which started in 1737. In fact, during the siege of Boston which forced the British evacuation on March 17th, 1776, General George Washington used "Boston" as the day's secret password and "St. Patrick" as the appropriate response.