Saint Patrick's Day is celebrated worldwide by those of Irish descent and increasingly by people of other ethnicities as well, notably in Argentina, Australia, New Zealand and North America. Celebrations are generally themed around all things Irish and, by association, the colour green. Both Christians and non-Christians celebrate the secular version of the holiday by wearing green, eating Irish food and/or green foods, imbibing Irish drink (such as Irish stout, especially Guiness, Irish Whiskey or Irish Cream) and attending parades, which have a particularly long history in the United States and in Canada.

As well as being a celebration of Irish culture, Saint Patrick's Day is a Christian festival celebrated in the Catholic Church, the Church of Ireland, and some other denominations. The day almost always falls in the season of Lent, and some bishops will grant an indult, or release, from the Friday no-meat observance when St. Patrick's Day falls on a Friday; this is sometimes colloquially known as a "corned-beef indult".

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.