It's St Patrick's Day — who was this saint and what does he have to ...

If you're seeing a lot of emerald green around you today you're not seeing things – it's just St Patrick's Day!

The annual occasion is celebrated all around with world with parades, shamrocks and — in the case of Chicago — by turning an entire river bright green.

Here's what you need to know about the day and the saint that it celebrates.

Who is St Patrick?

St Patrick is a Catholic saint, credited with bringing Christianity to Ireland in the 5th century.

He's known, primarily, from his two written works:

Confessio, which is a spiritual autobiographyLetter to Coroticus, which is a denunciation of the British mistreatment of the Irish.When and where was he born?

While exact dates vary, most historians agree that St Patrick was born between the late 300s and the mid-400s.

Despite being such a important figure in Irish culture, St Patrick was most likely born in Britain, near the southern border between modern Wales and England, according to medieval Ireland historian Lisa Bitel.

What is St Patrick the patron saint of?

St Patrick is the patron saint of Ireland.

What did he do to become a saint?

In his memoir, St Patrick describes being brought to Ireland as a slave, where he spent six years tending sheep in the west of Ireland.

He eventually escaped and headed back to Britain, where he details in Confessio that he had a dream in which the "voice of the Irish" encouraged him back to Ireland to convert as many people as possible to Christianity.

It wasn't an easy task, with the high kings of Ireland and pagan high priests resisting his efforts.

However, as his memoir says, he was eventually able to fuse Irish culture into Christianity, earning him his sainthood.

Chicago river dyed bright green in honor of St. Patrick's DayWhy is St Patrick's day on March 17?

That's supposedly the day Patrick died, back in the 5th century. 

When did the world start celebrating St Patrick's Day?

While people in Ireland have been celebrating the day for centuries, it was immigrants who brought the celebration to countries far and wide.

Irish immigrants in the United States were particularly influential, with the first St Patrick's Day parade held there in Boston in 1737.

New York had its first St Patrick's Day parade in 1762.

The mid-1800s saw a wave of people leaving Ireland — many due to the potato famine and resultant political unrest — to relocate in places such as the US and Australia, bolstering the celebrations even further.

Today, St Patrick's Day is a public holiday in the Republic of Ireland, Northern Ireland, the Canadian province of Newfoundland and Labrador (for provincial government employees), and the British Overseas Territory of Montserrat.

However, in many countries, St Patrick's Day is marked by parades, parties and plenty of hijinks.

What does St Patrick have to do with snakes?

One of the most-well-known legends about St Patrick is that he drove the snakes out of Ireland, like some kind of reptilian Pied Piper.

But that's not true because there were no snakes in pre-modern Ireland.

"At no time has there ever been any suggestion of snakes in Ireland, so [there was] nothing for St Patrick to banish," the National Museum of Ireland's Nigel Monaghan told the National Geographic in 2018.

Scientists think that, during the most recent Ice Age, Ireland became too cold for reptiles.

And, by the time things warmed up, rising waters from melting glaciers had cut the land link between Ireland and Britain before snakes could migrate from the UK.

This "miracle" was probably plagiarised from another saint and eventually infused into St Patrick's story, Ms Bitel said.

Another myth is of the shamrock — or the Irish seamróg — which St Patrick supposedly used as a symbol of the Christian Trinity when he preached to the Irish.

But Patrick never mentions shamrocks in his writings.

In fact, the first time a shamrock was connected to the saint was by an English visitor to Ireland in 1684.

He wrote that, on Saint Patrick's feast day, "the vulgar superstitiously wear shamroges, 3 leav'd grass, which they likewise eat (they say) to cause a sweet breath."

AP/ABC