The (Christian) holiday is named after Saint Patrick (AD 387-461), the most commonly recognized of the patron saints of Ireland. However, he is not from Ireland! He was born into a wealthy Roman-British family.
He was also know for wearing BLUE not GREEN.
So, you question where the green came into play? Well, he did use the shamrock to explain the the Holy Trinity to pagans. He also wore a shamrock which is how the color green became associated.
How come the holiday falls on March 17th? Well, that is the day that Saint Patrick died.
Now, I am sure that you are wondering how the drinking became to be a popular commodity to have on this day......
For those of you who aren't religious and don't know, but St. Patrick's Day always falls during Lent. Lent is the preparation of of the death and resurrection of Jesus. During this time believers prepare themselves through prayer, repentance, almsgiving, and self-denial - in other words they normally give up things such as drinking. However, on St. Patrick's Day the Lenten restrictions of fasting and drinking are lifts; therefore, you have an excuse to get drunk!