St. Patrick was a real person. He lived sometime in the fifth century A.D. and was British, not Irish. He ended up in Ireland because he was kidnapped and sold into slavery.
After seven years of slavery, he escaped back to Britain, but once free, he "felt the call of God to return to Ireland and was a missionary there for the rest of his life," says Philip Freeman, professor of classics at Luther College in Decorah, Iowa, and author of The World of St. Patrick.
Sometime after his death, thought to be on March 17, he was named the patron saint of Ireland for bringing Christianity to the previously pagan region.
So how did the holiday make its way to America and turn into a party?
When Irish soldiers and immigrants came to the U.S., they began to use the March 17 holiday to celebrate their homeland.
"The first St. Patrick's Day parades or celebrations go back to the 18th century," says Patrick Tally, professor of history at the University of Colorado Boulder, and "the Irishmen within the British Army in America."
In the 19th century, millions of Irish immigrated to the U.S. Like the soldiers who preceded them, they wanted to remember the country they'd left behind.