Though St. Patrick's Day originated in Ireland, the parades, parties, and practice of dyeing rivers green is a purely American tradition and celebration of Irish-American pride.
The holiday was first celebrated in 18th- and 19th-century Ireland as a small religious celebration in honor of an important figure in Irish history. Most people observed by going to church.
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?
The Greening of America
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.
"It was really in the early 19th century that you start getting municipal celebrations of St. Patrick's Day in places like Boston or New York, and they spread to other places where there's a large Irish population," he says. "As Irish Americans become politically powerful in big cities, cities themselves begin to back St. Patrick's Day celebrations."
St. Patrick's Day gained popularity with non-Irish Americans only "in the latter half of the 20th century," says Tally, when holidays began to be marketed more aggressively in the thriving post-WWII economy.
Still, it took several decades for the holiday to broaden its appeal.
"Even when I was a kid in the '70s, and I grew up in New England, it tended to be Irish people [who celebrated]," he says.
Large, secular celebrations of St. Patrick's Day appeared in Ireland only after they'd become popular in the United States. Because Ireland's parades were inspired by American ones, they represent a "kind of reverse migration," says Tally.
"They've imported the American celebration," he says.
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