Beer is green, the Chicago River is green and Niagara Falls will be green tonight. How did green get to be part of celebrating St. Patrick? Here is the answer from Wikipedia:
Green first shows up in the 11th century pseudo-historical book The Book of the Taking of Ireland. Goídel Glas, who was the son of Scota and Niul, was bitten by a snake and was saved from death by Moses placing his staff on the snakebite. As a reminder of the incident he would retain a green mark that would stay with him and he would lead his people to a land that would be free of snakes. This is emphasized in his name Goídel which was anglicised to the word Gaelic and Glas which is the Irish word for green.
That's quite a remarkable story.
The Duke and Duchess of Cambridge hand out shamrocks on this day to the Irish Guards. They traditionally attend a parade of the Irish Guards: William is Colonel of the Irish Guards.
The rest of us will indulge in wearing and drinking green.