St. Patrick is the patron saint of Ireland and has been honored in America since the first days of our nation. The most notable observance is the annual St. Patrick’s Day parade in New York City.
According to Wikipedia, The Feast of Saint Patrick, is a cultural and religious celebration occurring on 17 March on the death date of the patron saint of Ireland, Saint Patrick (c. AD 385–461).
It is customary to wear shamrocks, green clothing and accessories on St Patrick’s Day. He is said to have used the shamrock, a three-leaved plant, to explain the Holy Trinity to the pagan Irish people.