How did St. Patrick’s Day go from a day of celebration in remembrance of the spread of Irish Christianity, to a day of massive amounts of green beer?

According to Wikipedia St Patrick was born of Roman-British decent. He was captured by the Irish as a slave. When he escaped he became a priest and returned to Ireland spreading the teachings of Christianity and the doctrine of the Holy Trinity. After about 30 years of evangelism he died on March 17, celebrated as the principal champion of Irish Christianity.

Now I understand there are holidays to celebrate one’s culture. But it seems that this seemingly important detail has gotten lost amongst the drunken banter.