Saint Patrick's Day is as good an excuse as any to drink Irish whisky and stout, dress in green, listen to Irish folk music and generally celebrate the man who, allegedly, banished all snakes from Ireland after they disturbed him while he was fasting. (There are indeed no snakes in Ireland, but this is because they never came back after the last ice age.)
Not that Saint Patrick's Day is entirely an Irish phenomenon. The first recorded Saint Patrick's Day Parade was not in Ireland but in America - in Boston, in 1737. Dublin didn't get its own parade for almost 200 years - and 18 of the 20 largest parades are in the USA.
Saint Patrick is of course the patron saint of Ireland (along with Brigid of Kildare and Columba). Funnily enough though, he might not actually have been Irish - some sources say he was born in Wales in 385AD while others think he was born two years later in Scotland!
When Patrick was 18, he was enslaved by a sheep farmer. He converted to Christianity, and when he escaped four years later, he moved to France where he spent a dozen years in a monastery before moving back to Ireland.
Saint Patrick's symbol is the shamrock (seamrog), which is, most likely, a type of clover. It was a sacred plant to the early Irish druids because it had three leaves, and the number three was sacred in Celtic religion. Sensibly enough, the Christians soon adopted it as a sacred symbol too. Saint Patrick used the shamrock's mystical triad of leaves to teach about Christianity - in particular the Trinity, the holy threesome of Father, Son and Holy Ghost.