St. Patrick's Day, Exposed!
While most people get ready to celebrate St. Patrick's Day by putting on that Dropkick Murphys tee they bought after seeing The Departed, we on Thrillist's illustrious and deep-pocketed Historical Investigations Squad have more important things to do, like uncovering the tightly guarded secrets of the man behind the holiday most bar owners call "pretty much the worst day of my life." So take a seat, friends, and pour yourselves a nominally Irish beverage, because your mind is about to be lustfully throttled with facts.

The origin story
Patrick grew up around 400 AD in Cumbria, Great Britain, on the Scottish border back when Britain was vaguely controlled by Rome. His Father was a deacon, and definitely named Calpurnius, which must've made deacon-ing teenagers a blast. When Pat was a teenager himself, he was captured by those dudes who took Liam Neeson's daughter and carried off to Ireland, where he was forced to herd sheep and remain ever vigilant against clever wolves dressed in their clothing. After only six years in captivity, he claimed he heard a voice telling him he'd soon go home and that a ship was ready, so he -- apparently quite easily -- ditched his master, traveled to a port 200mi away, boarded a ship, and "after various adventures," returned home to his family. It's also unclear whether the voice he heard was just that of some dude who knew the ferry schedule.
Once home, Patrick found it awkward and embarrassing to be around a man named Calpurnius, so he claims to have had visions that told him he was to go back to Ireland, but not until he was an ordained bishop. Fast-forward however long it takes to become an ordained bishop (three months?), and all of a sudden he is headed back to Ireland to convert those pagan, druid-worshipping beasts into Christians. In this, Patrick was allegedly successful, and wrote that he'd "baptized thousands of people," ordained priests to lead new Christian communities, and even "converted wealthy women into nuns," as one is wont to do.
But life wasn't such a breeze, even for a bishop returning to a land where he was, for six years, held as a sheep-herding slave. Unlike that scene in Pretty Woman where Julia Roberts gets revenge on that saleswoman who embarrassed her when she was dressed in her prostie-clothes, putting on a fancy dress and hat didn't necessarily keep Patrick out of harm's way. In fact, he writes that, at one point, he was beaten, robbed, and put in chains awaiting execution. And on another occasion he stood trial for some sort of charge of impropriety or misuse of funds, and was forced to recount all the things he did for free, mentioning that "he returned gifts wealthy women gave him, did not accept payment for baptisms, nor for ordaining priests, and paid for many gifts to kings and judges." So basically, the Catholic Accounting Department flagged his Concur statements because his receipts seemed shady. If this were US Weekly, this would be filed under the section "Saints are just like us!"

On shamrocks, snakes, and living walking sticks
There are many legends and symbols associated with Saint Patrick. So let's just knock them out, one by one:
Shamrocks: Yes, Patrick did use the three-leafed shamrock to teach the Irish about the Holy Trinity, and how each leaf represented the Father, the Son, and the Holy Spirit. But (!) shamrocks were allegedly sacred anyway, because green meant rebirth and eternal life, three was a sacred number in Irish peoples' weird pagan religion before Christianity, and -- in that religion -- they worshipped a bunch of "Triple Goddesses," one of whom was apparently named "the Morrigan," or "phantom queen" and often appeared in the form of a crow flying above warriors before battle. Or as an eel. Or a wolf. Or a damn cow. Apparently Irish mythology wasn't very strict about the parameters of the Morrigan's shapeshifting.
Kicking all the snakes out of Ireland: Legend has it that Saint Patrick freaked out after a 40-day fast on a hill and chased all the snakes into the sea. But scientists seem to think otherwise, especially because post-glacial Ireland never actually had any snakes. They do have something called a slow worm, which is more of a gross, ugly, legless lizard, but apparently it was deliberately introduced in the 1960s, and Patrick would have had trouble chasing it down then, as he would've been 1,350 years old and had bad arthritis in his left knee. Some scholars believe that the references to snakes actually mean the "serpent symbolism of the Druids," and that Patrick "chasing the snakes out of Ireland" had less to do with actual snakes, and more to do with chasing out an old, weird religion heavily invested in the crow/bat/cow/wolf Morrigan.
His walking stick grows into an actual, living tree: Come on.

On his death and why it became a party involving green beer
Though Patrick died on March 17th in 460 AD, it wasn't until the early part of the 17th century that his death became a "feast day," and this was due in large part to the Irish Franciscan friar Luke Wadding, who had sway in Rome and loved parties and feasts and the like, despite every painting of him portraying a scary bald man frowning.
Although the original color associated with the day was blue, green became more and more synonymous with it as the stories of his teaching using the shamrock grew. And then, during the 1798 rebellion, Irish soldiers wore full green uniforms on March 17th in hopes of catching public attention, which -- as a marketing scheme -- was genius. Soon "the wearing of the green" on that day was everywhere, as it began to stand for both Irish nationalism and religious education.
In 1903, Ireland officially made Saint Patrick's Day a public holiday, and even required that pubs be closed during that day, but they repealed that in the 1970s, when it became clear that that was the stupidest idea ever.
During the mid-1990s, the Republic of Ireland set up a group called the St. Patrick's Festival whose aim was to, essentially, figure out ways to exploit St. Patrick's Day's international appeal for Ireland's financial gain. Or, as they explained it, "provide the opportunity and motivation for people of Irish descent" (and those who sometimes wish they were Irish) "to attend and join in the imaginative and expressive celebrations."
1996 was their first Festival, held on St. Patrick's Day. The next year it became a three-day event, and then it went to four, and by 2006, it was five days long, and seeing close to one million visitors. And then everyone in Ireland became rich as the economy grew, and so they put all their money into speculating on real-estate growth, and then the housing bubble burst, and, well... moving on!

On our own very special holiday
In the US, St. Patty's has been celebrated in certain Irish-American circles since the 18th century. The Charitable Irish Society of Boston organized an observance when we were still 13 colonies, and in NY, there was a parade in 1766 put on by Irish soldiers in the British army. In 1780, George Washington even gave his troops a holiday on March 17th, as "an act of solidarity with the Irish in their fight for independence." Further evidence that George Washington was the coolest stone-cold British-hater ever.
During the 19th century, as parades grew and people got more liquored up, scholars point out that the rhetoric had shifted away from just remembering dear old Ireland and more towards the mutual Irish and American "hatred sentiments" felt towards "British oppression and resistance." So basically, Irish people and Americans joined together in their mutual hatred of the British, and, in doing so, formed closer bonds themselves. U-S-A! U-S-A!
In the 20th century, although it's only technically a legal holiday in Suffolk County, MA (obviously) and Chatham County, GA (less obviously), more and more towns and cities and people started to hop on the St. Patty's day wagon, mostly because said wagon was filled with booze and other stuff dyed green.
The idea of drinking green beer dates back to 1952, when students at Miami of Ohio started doing it; they also apparently wore all green, and would (kind of terrifyingly) "affectionately pinch" anyone who wasn't wearing the right color. Seattle paints the traffic stripe of their parade route green. Chicago famously dyes its river green and has ever since 1962, when sewer workers used green dye to check for sewer discharges and got the idea because they needed something positive to look forward to, what with being sewer workers and all. Savannah dyes its downtown fountains green. Single aunts in New England plant peas. But perhaps our best example of just what St. Patrick's Day in the US means in the 21st century comes from Butte, Montana, where the parade doubles the city's population for the day, though "there currently is not an open container law in Butte and the event often becomes rowdy." Éirinn go Brách, indeed, friends. Éirinn go Brách.