The St. Patrick mythology includes the claim that he banished snakes from Ireland.
It’s true no snakes exist on the island today. But they never did. Ireland, after all, is surrounded by icy ocean waters—much too cold to allow snakes to migrate from Britain or anywhere else. But since snakes often represent evil in literature, when Patrick drives the snakes out of Ireland, it is symbolically saying he drove the old, evil, pagan ways out of Ireland.
The snakes myth and others—such as Patrick using three-leafed shamrocks to explain the Holy Trinity (Father, Son, and Holy Ghost)—were likely spread by well-meaning monks centuries after St. Patrick’s death.
Saint Patrick Day was placed on the 17th of March because the festival was designed to coincide, and, it was hoped, to replace the Pagan holiday known as Ostara; the second spring festival which occurs each year, which celebrates the rebirth of nature, the balance of the universe when the day and night are equal in length, and which takes place at the Spring Equinox (March 22nd this year). In other words, Saint Patrick’s Day is yet another Christian replacement for a much older, ancient Pagan holiday; although generally speaking Ostara was most prominently replaced by the Christian celebration of Easter (the eggs and the bunny come from Ostara traditions, and the name “Easter” comes from the Pagan goddess Eostre).
But now I think of it more as a day to celebrate the Irish in you, have some yummy corned beef and cabbage, and get shitfaced.