Saint Patrick was the patron saint and national apostle of Ireland who is credited with bringing christianity to Ireland. Most of what is known about him comes from his two works, the Confessio, a spiritual autobiography, and his Epistola, a denunciation of British mistreatment of Irish christians. Saint Patrick described himself as a "most humble-minded man, pouring forth a continuous paean of thanks to his Maker for having chosen him as the instrument whereby multitudes who had worshipped idols and unclean things had become the people of God."
Saint Patrick's Day has come to be associated with everything Irish: anything green and gold, shamrocks and luck. Most importantly, to those who celebrate its intended meaning, St. Patrick's Day is a traditional day for spiritual renewal and offering prayers for missionaries worldwide.
So, why is it celebrated on March 17th? One theory is that that is the day that St. Patrick died. Since the holiday began in Ireland, it is believed that as the Irish spread out around the world, they took with them their history and celebrations. The biggest observance of all is, of course, in Ireland. With the exception of restaurants and pubs, almost all businesses close on March 17th. Being a religious holiday as well, many Irish attend mass, where March 17th is the traditional day for offering prayers for missionaries worldwide before the serious celebrating begins.
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Text courtesy of www.st-patricks-day.com