Saint Patrick’s Day (17 March) is a Roman Catholic feast day which celebrates Saint Patrick (386-493), the patron saint of Ireland. It is a national holiday in the Republic of Ireland, Northern Ireland, the overseas territory of Montserrat and the Canadian province of Newfoundland and Labrador.

Saint Patrick’s Day is celebrated worldwide by Irish people and increasingly by many of non-Irish descent. The oldest and largest parade in the world is held in New York City. Parades also take place in Dublin and in most other Irish towns and villages. Other large parades include those in Savannah, Georgia, Manchester, New Orleans, Montreal, and Boston. Large parades also take place in other places, including London, Paris, Rome, Munich, Moscow, Beijing, Hong Kong, Singapore, Sydney, Copenhagen, and throughout the Americas.

As well as being a celebration of Irish culture, Saint Patrick’s Day is a Christian festival celebrated in the Roman Catholic Church, the Church of Ireland (among other churches in the Anglican Communion) and some other denominations. The day always falls in the season of Lent and sometimes during Holy Week. In church calendars (though rarely in secular ones) Saint Patrick’s Day is moved to the following Monday when it falls on a Sunday. If it falls in Holy Week, it is moved to the second Monday after Easter. It is traditional for those observing a lenten fast to break it for the duration of Saint Patrick’s Day.[1]

In many parts of the U.S., Britain, and Australia, expatriate Irish, those of Irish descent, and ever-growing crowds of people with no Irish connections but who may proclaim themselves "Irish for a day" also celebrate St. Patrick’s Day, usually by drinking alcoholic beverages (lager dyed green, Irish beer such as Murphys, Smithwicks, Harp or Guinness, or Irish whiskey, Irish Coffee or Baileys Irish Cream) and by wearing at least one article of green-colored clothing. Former New York City Mayor Ed Koch once proclaimed himself "Ed O’Koch" for the day and is one of the most famous people of non-Irish descent to publicly revel on the holiday.

[Source: Wikipedia]