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Cut and pasted from Wikipedia:
Saint Patrick's Day is a cultural and religious holiday celebrated on 17 March.
It commemorates Saint Patrick (c. AD 387–461), the most commonly recognised of the patron saints of Ireland, and the arrival of Christianity in Ireland. Saint Patrick's Day was made an official feast day in the early seventeenth century, and has gradually become a celebration of Irish culture in general.
The day is generally characterised by the attendance of church services, wearing of green attire and the lifting of Lenten restrictions on eating, and drinking alcohol, which is often proscribed during the rest of the season.
Saint Patrick's Day is a public holiday in the Republic of Ireland, Northern Ireland, Newfoundland and Labrador and in Montserrat. It is also widely celebrated by the Irish diaspora, especially in places such as Great Britain, Canada, the United States, Argentina, Australia, and New Zealand, among others. Today, St. Patrick's Day is probably the most widely celebrated saint's day in the world.