Saint Patrick's Day also known as the Feast of Saint Patick occurs each year on the Seventeenth of March (17 March). The day commemorates Saint Patrick and the arrival of Christianity in Ireland. Public parades, festivals and the use of the colour green and shamrocks are typical of St Patrick's Day. Celebrations occur in Ireland and most other countries which have a population with Irish ancestory such as the Unites States, Canada and Australia.
For more information, go here: https://en.wikipedia.org/wiki/Saint_Patrick%27s_Day