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 worshiped. Driving the snakes from Ireland was probably symbolic of putting an end to that pagan practice. While not the first to bring Christianity to Ireland, it is Patrick who is said to have encountered the Druids at Tara and abolished their pagan rites. The story holds that he converted the warrior chiefs and princes, baptizing them and thousands of their subjects in the “Holy Wells” that still bear this name.
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.
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!
– from www.st-patricks-day.com