In the 4th century, Patrick, a 16 year old British boy born to an aristocratic family, was kidnapped by Irish raiders. He was sent across the sea to Ireland, enslaved and made to tend sheep. Several years later he was able to escape and returned to Britain where he became a priest. Eventually he returned to Ireland and spent the rest of his life spreading the teachings of Christianity to pagans. The story goes he used the shamrock to explain the Christian concept of Trinity to the Irish. For thirty years he baptized pagans and converted them to Christianity, he established churches, schools and monasteries. After a hard and arduous life, Patrick died on March 17, 461. His life and good works were forgotten for centuries but slowly he was immortalized and became the patron saint of Ireland.
On March 17th, people all over the world honor the patron saint of Ireland, Saint Patrick with parades, wearing of the green, and imbibing in green beer. However, some people still attend church and celebrate St Patrick's Day as a religious observance. Reuzeit Emporium has great antique St. Patrick's Day postcards for sale, check them out.