NEW YORK (AP) — If it’s March, and it’s green, it must be St. Patrick’s Day.

The day honoring the patron saint of Ireland is a global celebration of Irish heritage. And nowhere is that more so than in the United States, where parades take place in cities around the country and all kinds of foods and drinks are given an emerald hue.

In fact, it was among Irish American communities that the day became the celebration it is, from its roots as a more solemn day with a religious observance in Ireland.

But even in America, it was about more than a chance to dye a river green (looking at you, Chicago) or just bust out a favorite piece of green clothing, it was about putting down roots and claiming a piece of the country’s calendar.

WHO IS ST. PATRICK AND WHY DOES HE EVEN HAVE A DAY?

Patrick was not actually Irish, according to experts. Born in the late fourth century, he was captured as an adolescent and ended up enslaved in Ireland. He escaped to another part of Europe where he was trained as a priest and returned to Ireland in the fifth century to promote the spread of Christianity.

Several centuries later, he was made a saint by the Catholic Church and like other saints had a day dedicated to him, which was March 17th. He became Ireland’s patron saint, and even when religious strife broke out between Catholics and Protestants, was claimed by both, says Mike Cronin, historian and academic director of Boston College Dublin.

HOW DID AN IRISH SAINT’S DAY BECOME AN AMERICAN THING?

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The short answer: Irish people came to America and brought their culture with them. St. Patrick’s Day observances date back to before the founding of the U.S., in places like Boston and New York City. The first parade was held in Manhattan in 1762.

While the day was marked with more of a religious framing and solemnity in Ireland until well into the 20th century, in America it became the cultural and boisterous celebration it is today, marked by plenty of people without a trace of Irish heritage.

It was because people in Ireland started seeing how the day was marked in the U.S. that it became more of a festival in the country of its origin rather than strictly a religious observance, Cronin says, pointing to the parades, parties and other festivities that are held.

Oh, and by the way, for those who like to shorten names: Use St. Paddy’s Day, not St. Patty’s Day. Paddy is a nickname for Pádraig, which is the Irish spelling of Patrick.

WHY IS IT SUCH A BIG DEAL TO CELEBRATE A HOLIDAY LIKE THIS?

Holidays aren’t simply days to watch bands go by, or wear a specific outfit or costume.

Being able to mark a holiday, and have others mark it, is a way of “putting down roots, showing that you’ve made it in American culture,” says Leigh Schmidt, professor in the Danforth Center on Religion and Politics at Washington University. “You’ve made your claim on that American calendar, in American civic life, by having these holidays widely recognized.”

The spread of St. Patrick’s Day celebrations in the U.S. was a way for Irish immigrant communities, who in the 19th century faced discrimination and opposition, to stake that ground, he says: “It’s a kind of immigrant Irish way of combating nativist antagonism against them.”

WHAT’S WITH FOUR-LEAF CLOVERS, ANYWAY?

A popular sight around the holiday is the shamrock, or three-leaf clover, linked to Ireland and St. Patrick.

The lucky ones, though, come across something that’s harder to find: a four-leaf clover. That’s because it takes a recessive trait or traits in the clover’s genetics for there to be more than the normal 3 leaves, says Vincent Pennetti, a doctoral student at the University of Georgia’s College of Agricultural and Environmental Sciences. He has been fascinated by the plants since high school.

Four-leaf clovers “are real. They are rare,” he says.

That doesn’t mean they can’t be found. People just have to keep their eyes open and “get really good at noticing patterns and breaks in the patterns, and they just start jumping out at you,” he says.

Katie Glerum finds them. The 35-year-old New York City resident says it’s not unheard of for her to be somewhere like Central Park and see one. She usually scoops it up and often gives it to someone else, to a positive response.

“If it happened every day, then I probably would be less excited about it,” she says. “But yeah, when it happens, it is exciting.”