As you are aware, Saint Patrick is the patron saint and national apostle of Ireland. He is credited with bringing Christianity to Ireland and for driving snakes out of the country. Whether this is true or not is another thing as there were most probably never any snakes there to begin with.
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, music and songs and Irish food and drink. Some communities even go so far as to dye rivers or streams green!
Pop along to The Grapes in Clifton village where they normally have Irish music on a Monday but will have a special line up on St Patricks Day or just pop along to any normal pub and have a pint or three of Guinness to celebrate before going home for a plate of champ!