The March 17 celebration started in 1631 when the Church established a Feast Day honoring St. Patrick. He had been Patron Saint of Ireland who had died around the fifth century—a whopping 12 centuries before the modern version of the holiday was first observed. But very little is known about who he actually was, according to Marion Casey, a clinical assistant professor of Irish Studies at New York University (and a regular marcher in the St. Patrick’s Day Parade in Manhattan).