I learned some history from this short piece on the origins of St. Patrick's Day. (Via Snazzyman - if you're not following him yet, click through that hat tip!)
Why the color of St. Patrick’s Day changed from blue to green
St. Patrick’s Day usually conjures images of partying, Catholicism, Irish nationalism and, perhaps most famously, the color green: green clothes, green shamrocks, green beer and green rivers. But the holiday used to be a solemn feast day when you’d be far more likely to see the color blue.
pbs.org