St. Patrick died more than 1,500 years ago on March 17. It's now the saint's designated feast day. According to the Catholic Encyclopedia, we have theologian Luke Wadding to thank for the celebration's permanence: "Each year Wadding kept the Feast of St. Patrick with great solemnity, and it is due to his influence ... that the festival of Ireland's Apostle was inserted on 17 March in the calendar of the Universal Church." While "great solemnity" has clearly not persisted, the date has held firm.