St. Patrick drove the snakes from Ireland, true. And for his troubles he was awarded his day of feasting. However, St. Patrick's Day is one of the days of feasting that falls during the season of Lent. As such, it's observers are allowed to take a break from Lent to honour St. Patrick.

Most people give up alcohol for Lent, and as such, the last time they really got a chance to party was on Fat Tuesday, the time when Mardi Gras is celebrated. So, by the time St. Patrick's Day rolls around, they're ready for a good liquoring up, for the next day, it's back to sobriety.

This explains why both Mardi Gras and St. Patrick's Day have turned into such wild parties and showcases of debauchery. Of course, don't try explaining this to your average fratboy, as he only cares that the beer is green and import beers are half off.