Saint Patrick's Day or the Feast of Saint Patrick (Irish: Lá Fhéile Pádraig, "the Day of the Festival of Patrick") is a cultural and religious holiday celebrated on 17 March. It is named after Saint Patrick, the most commonly recognised of the patron saints of Ireland.
Saint Patrick's Day was made an official feast day in the early seventeenth century and is observed by the Catholic Church, the Anglican Communion (especially the Church of Ireland), the Eastern Orthodox Church and Lutheran Church.
For Christians, the day commemorates Saint Patrick and the arrival of Christianity in Ireland. However, it has also become a celebration of Irishness and Irish culture in general, source Wikipedia.