Have you ever wondered how St. Patrick's started? Well it appears that St. Patrick's Day actually started as a roman catholic holiday to recognize St. Patrick (who died on the 17th of March) who entered the priesthood at the age of twenty after escaping from slave traders (http://www.huffingtonpost.com/entry/st-patricks-day-2016_us_56e99e81e4b0b25c91842a6b).
This holiday celebrates the heritage and culture of the Irish. Celebrations include parades, the wearing of green, and for some going to church. The Shamrock became the symbol of St. Patrick's Day because it is said that St. Patrick used the three leaf plant to help explain the Holy Trinity: The Father, The Son, The Holy Ghost. I can see how hard this would be as one would try to explain the three as one. The use of the three leaf clover would help those to understand how this could be.
Lent is often lifted on this day as one usually has a grand feast of corned beef and cabbage with green beer. Seeing how this holiday started as a Roman catholic holiday, it came to United States with Irish immigrants as a way of keeping their heritage even though they were no longer in Ireland.
So, make sure you wear your green today so the leprechauns won't pinch you. I hope everyone has a great day and that there is not much trouble from all the drinking...lol.
Have a great day!