The Wearing Of The Green
Legends tell of the young Bishop Saint Patrick, who plucked a simple, humble shamrock from the ground somewhere on the lush green hillsides of County Armagh back in the year 432 AD. He used it to explain the Mystery of the Blessed Trinity to the Ancient High Kings of Ireland, making the Shamrock an important symbol as Ireland's National Emblem. It is also said that Saint Patrick drove the snakes from Ireland into the sea. St. Patrick is named the patron Saint for bringing Christianity to Ireland, and introducing the Irish people to Baptism.
That is why March 17th is now celebrated as the "Feast Day Of Saint Patrick", when Irish people from all over the world traditionally wear the shamrock on their chests, hence the phrase "The Wearing Of The Green". Saint Patrick's death on March 17, in about 461, has been observed in America since colonial days.