Saint Patrick's Day is a public holiday in the Republic of Ireland, Northern Ireland, the Canadian province of Newfoundland and Labrador, and the British Overseas Territory of Montserrat. It is also widely celebrated by the Irish diaspora around the world, especially in Great Britain, Canada, the United States, Argentina, Australia and New Zealand.
On St Patrick's Day it is customary to wear shamrocks and/or green clothing or accessories (the "wearing of the green"). St Patrick is said to have used the shamrock, a three-leaved plant, to explain the Holy Trinity to the pagan Irish. This story first appears in writing in 1726, though it may be older.