All you need to know about St Patrick's Day

The day honouring 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.

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?

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 US, 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 US 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.

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 three leaves, says Vincent Pennetti, a doctoral student at the University of Georgia's College of Agricultural and Environmental Sciences.

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.

Australian Associated Press