Saint Patrick's Day has come to be associated with everything Irish: anything green and gold, shamrocks and luck. Most importantly, to those who celebrate its intended meaning, St. Patrick's Day is a traditional day for spiritual renewal and offering prayers for missionaries worldwide.
Shamrocks have been symbolic of many things over the years. According to legend, the shamrock was a sacred plant to the Druids of Ireland because its leaves formed a triad, and three was a mystical number in the Celtic religion, as in many others. St. Patrick used the shamrock in the 5th century to illustrate the doctrine of the Holy Trinity as he introduced Christianity to Ireland. The shamrock became symbolic in other ways as time went on. In the 19th century it became a symbol of rebellion, and anyone wearing it risked death by hanging. It was this period that spawned the phrase "the wearin' o' the green".
Today, the shamrock is the most recognized symbol of the Irish, especially on St. Patrick's Day, when all over the world, everyone is Irish for a day! So, why is it celebrated on March 17th? One theory is that that is the day that St. Patrick died. Since the holiday began in Ireland, it is believed that as the Irish spread out around the world, they took with them their history and celebrations. The biggest observance of all is, of course, in Ireland. With the exception of restaurants and pubs, almost all businesses close on March 17th. Being a religious holiday as well, many Irish attend mass, where March 17th is the traditional day for offering prayers for missionaries worldwide before the serious celebrating begins.
In American cities with a large Irish population, St. Patrick's Day is a very big deal. Big cities and small towns alike celebrate with parades, "wearing of the green," music and songs, Irish food and drink, and activities for kids such as crafts, coloring and games. Some communities even go so far as to dye rivers or streams green!