Saint Patrick's Day (Irish: Lá ’le Pádraig or Lá Fhéile Pádraig), colloquially St. Paddy's Day or Paddy's Day, is an annual feast day which celebrates Saint Patrick (circa 385–461 AD), one of the patron saints of Ireland, and is generally celebrated on March 17.
The day is the national holiday of Ireland. It is a bank holiday in Northern Ireland and a public holiday in the Republic of Ireland and Montserrat. In Canada, Great Britain, Australia, the United States and New Zealand, 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 for Roman Catholics in Ireland. The feast day usually falls during Lent; if it falls on a Friday of Lent (unless it is Good Friday), the obligation to abstain from eating meat can be lifted by the local bishop. The date of the feast is occasionally, yet controversially, moved by church authorities when March 17 falls during Holy Week; this happened in 1940 when Saint Patrick's Day was observed on April 3 in order to avoid it coinciding with Palm Sunday, and happened again in 2008, having been observed on 15 March.
Taken from wikipedia.com