St. Patrick's feast day was placed on the universal liturgical calendar in the Catholic Church due to the influence of the Waterford-born Franciscan scholar Luke Wadding in the early part of the 17th century, although the feast day was celebrated in the local Irish church from a much earlier date. St. Patrick's Day 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, the obligation to abstain from eating meat does not bind as St.Patrick's day in Ireland is a first class feast, thus removing the obligation to fast or abstain. The church calendar avoids the observance of saints' feasts during certain solemnities, moving the saint's day to a time outside those periods. St. Patrick's Day is very occasionally affected by this requirement. Thus when 17th of March falls during Holy Week, as in 1940 when St. Patrick's Day was observed on 3 April in order to avoid it coinciding with Palm Sunday, and again in 2008, having been observed on 15 March. St. Patrick's Day will not fall within Holy Week again until 2160 - when it will fall on the Monday before Easter.

When 17 March falls on a Sunday, church calendars (though rarely secular ones) move Saint Patrick's Day to the following Monday — and when the 17th falls during Holy Week (very rarely), the observance will be moved to the next available date or, exceptionally, before holy week. The public holiday in Ireland occurs on 17 March, unless that date occurs at the weekend being fixed on the State calendar. In North American communities with large populations of persons of Italian descent, St. Patrick's Day celebrations are often combined with those of Saint Joseph's Day, which takes place on 19 March, two days after St. Patrick's Day.

The New York parade has become the largest Saint Patrick's Day parade in the world. In a typical year, 150,000 marchers participate in it, including bands, firefighters, military and police groups, county associations, emigrant societies, and social and cultural clubs, and 2 million spectators line the streets. The parade marches up 5th Avenue in Manhattan and is always led by the U.S. 69th Infantry Regiment when the Commissioner of the parade asks the Commanding Officer if the 69th is ready. To which the response is "The 69th is always ready." New York politicians - or those running for office - are always found prominently marching in the parade. Former New York City Mayor Ed Koch once proclaimed himself "Ed O'Koch" for the day, and he continues to don an Irish sweater and march every year up until 2003, even though he is no longer in office.

The parade is organized and run by the Ancient Order of Hibernians. For many years, the St. Patrick's Day Parade was the primary public function of the organization. On occasion the order has appointed controversial Irish republican figures (some of whom were barred from the U.S.) to be its Grand Marshal. The parade has also drawn controversy for many years for its exclusion of openly gay organizations.

The New York parade is moved to the previous Saturday (16 March) in years where 17 March is a Sunday. The event also has been moved on the rare occasions when, due to Easter falling on a very early date, 17 March would land in Holy Week. This same scenario arose again in 2008, when Easter fell on 23 March. The festivities went ahead on their normal date and had record viewers.