Saint Patrick's Day, now celebrated in Ireland, the United Kingdom, the United States, Australia, and New Zealand, is an unusual blend of Christian and pagan customs, and a celebration of the venerated Catholic saint and missionary, Patrick.

Patrick was born in Wales in 385. When he was sixteen, he was taken from his home and sold into servitude in Ireland, where he spent six years working as a shepherd. During this time, he became a devout Christian. Upon escaping from slavery, he went to Gaul, where he studied Christian doctrine for fifteen years and was eventually ordained to the Catholic priesthood. He subsequently returned to Ireland as a missionary.

Contrary to popular belief, Patrick did not introduce Christianity to Ireland. There were already Christians living in Ireland, but they were a very small minority, with most of the indigenous Irish population following Celtic-pagan beliefs. Having already lived among the common Irish people for six years, Patrick was ideally suited to bring Christianity to Ireland on a large scale.

In his missionary efforts, Patrick used his knowledge of Irish culture and pagan belief to explain Christianity in terms that the people would understand. The shamrock, for instance, was already an important symbol to the people, signifying spring and rebirth. Patrick employed the shamrock to represent the Trinity: three separate pieces all part of a whole. Patrick's newly-won converts adopted the shamrock as the symbol of their Christianity and wore it on their clothes; later this transformed into the custom of wearing green to celebrate Saint Patrick's Day.

Another prevalent Celtic symbol was fire. The Celts used bonfires to honor their gods, so Patrick adopted this custom into the Catholic celebration of Easter, and early Irish Christians celebrated the holy day with fires and revelry. Because the Irish pagans also worshiped the sun, he superimposed an orb over the cross to create a new icon of the Irish Catholic faith. This emblem is now recognized as the Celtic cross.

One common myth about Patrick is that he drove all the snakes out of Ireland. Actually, there were never very many snakes in Ireland to begin with. This legend comes from an incident that is purely symbolic: Patrick stood on a hilltop (which now bears his name), held up his wooden staff, and "banished the snakes" from Ireland. The "banishing of the snakes" represented the eradication of pagan doctrines from Ireland. Within two centuries, Patrick's vision had been realized, and Christianity had triumphed.