Wearing of the Green
“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.
The colour green has been associated with Ireland since at least the 1640s, when the green harp flag was used by the Irish Catholic Confederation. Green ribbons and shamrocks have been worn on St Patrick’s Day since at least the 1680s.” – Wiki