Monday, March 17 is St. Patrick's Day, celebrating all things Irish in the U.S. and around the world. But it wasn't always a day for partying.

Every year thousands of revelers fill the streets in the U.S., Ireland and countries worldwide to celebrate Saint Patrick’s Day, but the real Saint Patrick wasn’t even Irish. NBC News’ Joe Fryer took a deeper look at Saint Patrick’s lasting influence and the history behind the holiday.

St. Patrick’s Day, the annual celebration of all things Irish, is being marked in cities across the country on Monday with boisterous parades and celebrations. New York City hosts one of the largest and oldest parades in the United States.