Saint Patrick's Day (Irish: Lá ’le Pádraig or Lá Fhéile Pádraig), colloquially St. Paddy's Day or Paddy's Day, is the feast day which annually celebrates Saint Patrick (385–461), one of the patron saints of Ireland, on March 17, the day on which Saint Patrick died. The day is the national holiday of the Irish people. It is a Bank Holiday in Northern Ireland, and a public holiday in the Republic of Ireland, Montserrat, and the Canadian province of Newfoundland and Labrador. In the rest of Canada, Australia, the United Kingdom and the United States, it is widely celebrated but is not an official holiday.

It became a feast day in the Roman Catholic Church due to the influence of the Waterford-born Franciscan scholar Luke Wadding in the early part of the 17th century, and is a holy day of obligation in the Roman Catholic church in Ireland.

[This is an excerpt from the Wikipedia article about St Patrick's Day.]