"Saint Patrick's Day (Irish: Lá Fhéile Pádraig) is a religious holiday celebrated internationally on 17 March. It  commemorates Saint Patrick (c. AD 387–461), the most commonly recognised of the patron saints of Ireland, and the arrival of Christianity in Ireland.[2] ...the day was made an official feast day in the early 17th century, and has gradually become a celebration of Irish culture in general.[4].
Originally, the colour associated with Saint Patrick was blue. Over the years the colour green and its association with Saint Patrick's day grew.^[18] Green ribbons and shamrocks were worn in celebration of St Patrick's Day as early as the 17th century.^[19] He is said to have used the shamrock, a three-leaved plant, to explain the Holy Trinity to the pagan Irish, and the wearing and display of shamrocks and shamrock-inspired designs have become a ubiquitous feature of the day." - courtesy of good ol' wikipedia.