The feast day of the patron saint of ireland. Great because you get off school and/or work (in Ireland at least) and you have an excuse to swear a lot, drink a lot and be patriotic to the emerald isle. The parades are brilliant craic and remember to wear green... =]
Stupid if you're not actually irish, only pretending. You're not irish if your great great great granny emigrated from dublin in 1709. =
"Are ye coming down to the Saint Patrick's Day parade in Cork?"
"Aye, what's it got for me?"
"Drink, girls and a load o' craic"