"The Feast of St. Patrick is celebrated by Roman Catholics, the Anglican Communion, and Lutherans on March 17. The day is also popularly celebrated, particularly in the U.S., by 'the wearing of the green,' with many people of Irish and other extractions wearing some item of green clothing. Parties featuring corned beef and cabbage, and even the drinking of beer dyed green with food coloring are also part of this celebration of Irish heritage."
The Films on Demand database has a more in-depth look at how St. Patrick's Day came to be. See:
St. Patrick: The Living Legend (1994, 52 min.)
"This documentary—filmed in Boston, California, Newfoundland, and, of course, throughout Ireland—explores the origins of Saint Patrick’s Day and how the tradition grew of celebrating him with parades and as a celebration of being Irish."