St. Patrick's Day.

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 (c. AD 385–461), the most commonly recognized of the patron saints of Ireland.
Saint Patrick's Day was made an official Christian 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. The day commemorates Saint Patrick, his death and entrance into heaven, and the arrival of Christianity in Ireland.  In addition, the Irish as well as Irish heritage and culture is celebrated in general. Celebrations generally involve public parades and festivals, céilithe, and the wearing of green attire or shamrocks. Christians also attend church services and the Lenten restrictions on eating and drinking alcohol are lifted for the day, which has encouraged and propagated the holiday's tradition of alcohol consumption.
Saint Patrick's Day is a public holiday in the Republic of Ireland^, Northern Ireland, Newfoundland and Labrador and Montserrat. It is also widely celebrated by the Irish diaspora around the world; especially in Britain, Canada, the United States, Argentina, Australia and New Zealand.

According to legend, St. Patrick used the 3-leaved shamrock to explain the Holy Trinity to Irish pagans.