For the venerated reality of St. Patrick's Day, the legend is that St. Patrick drove all the venomous snakes out of Ireland. This, in fact, was merely an allegory for St. Patrick driving out all of the heathen and pagan practices from Ireland. There is no record of St. Patrick driving out actual snakes, because actual snakes were not known to inhabit the region.
St. Patrick's Day proper was celebrated as a day of Christian obligation for Irish Catholics as early as the ninth and tenth century A.D. and was written into the liturgy of the Anglican Church by the 1600's.
The local draw for St. Patrick's Day is to visit your local Irish pub and watch a Boston Celtics game with your buddies. Invariably, there will be the obligatory green beer, corned beef and cabbage, and soda bread. If perhaps you find yourself straggling home late Sunday night, please be advised that the local gendarmes will be watching for folks who may have had little too much Sunday.
Heathen practices aside, the purpose of St. Patrick's Day was a manifesto of a cleansing of pagan practices that had mixed with Christian practices. Much like the ancient kings mentioned in Bible times, St. Patrick was doing exactly what good kings did when their subjects had fallen away from observing the "law."
Yet, instead, St. Patrick's Day became steeped in tradition and celebration, with feasting and drinking. What had originally been purposed as a day of celebration of some "church housecleaning," became one of feasting and drinking because the wife had cleaned the house prior to your guests arriving.