St. Patrick’s Day originated as an Irish Celebration of an early missionary
Over time, the holiday moved away from its religious roots and became associated with Irish pride. Ireland is a very green country, so people started wearing green on St. Patrick’s Day. (In many schools, a child not wearing green on St. Patrick’s day will be pinched by his friends.) The four-leaf-clover replaced the three-leafed-clover and became a symbol of luck rather than the Trinity. There are often parades in cities with big Irish populations.