1) People won't need to call in hungover (er, "sick"). Many people use St. Patty's Day to drink themselves into a stupor. This is fact. Thus, I have to assume that a number of people call in sick the day after. That means that the rest of us have to work extra hard to make up for their absences, and that just blows.
2) It's a wasted day. Those that DO come in hungover are clearly not going to be on their A game, but don't want to use a sick day or paid time off when they can just slack off on the company's dime. Again, those of us who didn't render ourselves useless may have to work harder (boooo-urns) or will just sit around talking about the adventures the others had the day before.
3) It's just time for a break. Many of us haven't had a holiday off since New Year's Day. It's about damn time, so why not the day after St. Patty's? Otherwise we have to wait until Memorial Day, which feels like for-ev-er. We're getting restless, which means productivity could be dropping. Why risk it?
4) I have two words: Spring Fever. The weather is finally getting nice enough to enjoy! Let us have a day to actually enjoy it instead of staring longingly out the window wishing we were enjoying it.
5) Not everyone is Irish, but most everyone can agree that a day off is a good thing. We can call it People Appreciation Day or Random Day Off Day. I don't care. I doubt anyone will care. Just do it.