St. Patrick is the patron saint and national apostle of Ireland.
He is most known as the saint who drove the snakes from Ireland.
St. Patrick's Day has been associated with anything Irish: green and gold and leprechaun and shamrocks.
He used the 3-leaved shamrock plant to explain the Holy Trinity to the Pre-Christian Irish.
Why celebrate on the 17th of March? There's a theory that he died on this day.
(source: http://www.st-patricks-day.com/about_saintpatrick.asp and http://www.en.wikipedia.org/)