Ah, yes, March 17th - when the Irish (and reasonable facsimilies) get together to celebrate and honor St. Patrick, who... uh...
Just who is this Saint Patrick fellow, anyway?
Well, almost fifteen centuries ago, a Gaelic family relocated to Britain, then part of the Holy Roman Empire. Their child was baptized Patricius, which meant "noble".
Patricius seems to have lived fairly comfortably until he was sixteen - when he was kidnapped. He worked as a slave until he was 22, herding sheep (which, as slavery goes, could have been worse...).
Having had enough of sheep, apparently, Patricius (ohhhh, let's just call him Patrick and get it over with) escaped, and eventually returned to Ireland to do missionary work for the Church. For the next forty years he traveled the countryside, preaching the faith and establishing churches and schools.
Legend has it that Patrick used the lowly shamrock to demonstrate the mystery of the Holy Trinity to the Irish. As God is three persons but one God, so then does the shamrock have three stems in one plant.
The word shamrock, by the by, is derived from the Irish seamrog, which means "summer plant" in Gaelic.
Another legend has it that Patrick drove the venomous snakes out of Ireland by beating on his drum (I've heard Irish bands that bad, myself). To this day there are no vipers on the Emerald Isle.
Patrick is alleged to have died on March 17, 493. He was later canonized upon confirmation of two miracles attributed to him, and became the patron saint of Ireland. To this day St. Patrick is a national hero to the Irish, who have embraced him and his symbol, the shamrock.