St. Patrick's Day. You know, Irish, Leprechauns, drinking...oh, and pinching people pinching peple who don't wear green. You see, a long time ago, some blokes on an island call Ireland decided to make green their national color because they were all redheads and green was the only color that they looked good in. Then they ran out of potatoes so a lot of them had to go to other places. And Saint Patrick's Day is a special day for them, so all the Irish people everywhere celebrate. Of course, I'm not Irish--I just get drunk at their parties.

[The explanation Gilgamesh has given is extremely simplified, not to mention politically incorrect in places.]