Legend has it that St. Patrick used the three-leaved shamrock to explain the concept of the Holy Trinity to the pre-Christian Irish, which led to the display and wearing of the shamrock, also known as "the wearing of the green," on St. Patrick's Day. Wearing green has since expanded to include dying your hair green; dressing in green from head to toe; wearing shamrock glasses, headbands, necklaces and stickers; and pretty much incorporating the color green into your dress, your food and anything else you can think of on March 17.

More info:

http://www.hometownannapolis.com/news/ent/2010/03/11-03/St-Patricks-Day-celebrations-galore.html