Many folk ask the question 'Why is the Shamrock the National Flower of Ireland?' The reason is that St. Patrick used it to explain the Holy Trinity to the pagans. Saint Patrick is believed to have been born in the late fourth century, and is often confused with Palladius, a bishop who was sent by Pope Celestine in 431 to be the first bishop to the Irish believers in Christ.
Snakes in Ireland?
Saint Patrick is most known for driving the snakes from Ireland. It is true there are no snakes in Ireland, but there probably never have been - the island was separated from the rest of the continent at the end of the Ice Age. As in many old pagan religions, serpent symbols were common and often worshipped. Driving the snakes from Ireland was probably symbolic of putting an end to that pagan practice.
Why Saint Patrick's Day?
Saint Patrick is the patron saint and national apostle of Ireland. St Patrick is credited with bringing Christianity to Ireland. Most of what is known about him comes from his two works; the Confessio, a spiritual autobiography, and his Epistola, a denunciation of British mistreatment of Irish Christians. Saint Patrick described himself as a "most humble-minded man, pouring forth a continuous paean of thanks to his Maker for having chosen him as the instrument whereby multitudes who had worshipped idols and unclean things had become the people of God."
Saint Patrick's Day has come to be associated with everything Irish: anything green and gold, shamrocks and luck. Most importantly, to those who celebrate its intended meaning, St. Patrick's Day is a traditional day for spiritual renewal and offering prayers for missionaries worldwide. So, why is it celebrated on March 17th? One theory is that that is the day that St. Patrick died. Since the holiday began in Ireland, it is believed that as the Irish spread out around the world, they took with them their history and celebrations.
The biggest observance of all is, of course, in Ireland. With the exception of restaurants and pubs, almost all businesses close on March 17th. Being a religious holiday as well, many Irish attend mass, where March 17th is the traditional day for offering prayers for missionaries worldwide before the serious celebrating begins.
In American cities with a large Irish population, St. Patrick's Day is a very big deal. Big cities and small towns alike celebrate with parades, "wearing of the green," music and songs, Irish food and drink, and activities for kids such as crafts, coloring and games. Some communities even go so far as to dye rivers or streams green!