I find the history of St. Patrick's Day interesting. The Shamrock was used by St. Patrick to teach the Holy Trinity. That green was not the originasl color associated with the day. It started as blue.
In the States, the familiar parades actually can be traced to the early 1770's as more like protest marches in Boston and they New Youk City a few years later. And that George Washington allowed a day off to his soldiers, I think it said 1770, as a show of solidarity with the Irish, who were also seeking independance.