March 17th is an Irish holiday honoring Saint Patrick, the missionary credited with converting the Irish to Christianity (in the A.D. 400's). Saint Patrick (real name: Maewyn Succat?) was kidnapped at the age of 16 by pirates and sold into slavery (I am not making this up). During his 6-year captivity (he worked as a shepherd), he found strength in his religious faith. 
 

After he escaped, Saint Patrick studied religion in Europe,then travelled to Ireland to spread the Christian word there. Apparently he won converts because he combined the old pagan customs with christian ones. He used the shamrock, which resembles a three-leafed clover, as a metaphor to explain the concept of the Trinity (father, son, holy spirit).
 

Legend has it that Saint Patrick drove all the snakes out of Ireland -- that they all went into the sea and drowned. Poor snakes. I don't know why he would want to do this, except that the snake was a revered pagan symbol, and perhaps this was a figurative tale alluding to the fact that he drove paganism out of Ireland.
 

In America, Saint Patrick's Day is a basically a time to wear green and party. The first American celebration of Saint Patrick's Day was in Boston, Massachusetts, in 1737. As the saying goes, on this day "everybody is Irish!" Over 100 U.S. cities now hold Saint Patrick's Day parades, the largest held in New York City.
 

Green is associated with Saint Patrick's Day because it is the color of spring, Ireland, and the shamrock. Leprechauns are also associated with this holiday, although I'm not sure why. Leprechauns of legend are actually mean little creatures, with the exception of the Lucky Charms guy. They were probably added later on because capitalists needed something cute to put on greeting cards.