How does Paris fit into the legend of Friday the 13th?

It depends if you are a conspiracy theorist, Christian or a feminist.

First, the conspiracy;

As host city to the violent end to the Knights of the Temple, Paris plays a not unimportant role in the making of Friday the 13th.

In 1312, bankrupt King Phillipe le Bel of France along with the corrupt Pope Clement V in Rome needed lots of money. Willing to work together, they looked around and saw that their own subjects, the Knights of the Temple, had plenty. However, getting the loot would not be easy as the Knights were one of the strongest and best armed organizations around. In fact, their main fortress served as Paris’ best guarded treasury and even housed, for a fee, the King’s own gold.

Fully Catholic, nominally under oath to the papacy and headquartered in Paris, the Knights of the Temple, also known as the Templars, were soldier-priests originally recruited throughout Europe to spearhead the Crusades.  Led by nobility, the rank and file Knights were mostly unemployed second or third sons of European gentry as under western tradition at the time, first sons inherited everything while younger brothers went to war or the church. In the case of the Knights of the Temple, it was both.

Officially known as the Poor Fellow-Soldiers of Christ and of the Temple of Solomon, the Templars were one of many Christian fighting orders formed to regain Jerusalem. These all-male Catholic fraternities grew at a time when Europe’s population was increasing, and rulers saw a need to occupy the growing demographic of unemployed young men. What better way than to send them off to smite the infidels in the Holy Land. The Knights of the Temple stood out amongst the other crusading orders for its vow of poverty but also, as the fighting carried on over the centuries, for their success in war and wealth.

Two knights on one horse symbolized their poverty

For almost two hundred years starting in 1119, the Knights of the Temple fought Saladin and other potentates while theoretically observing their monastic vows. (Knights swore not just to a life of poverty, but to one of chastity and bravery as well. In fact, they swore to obey so many rules such as eating in silence, never letting the Templar flag fall in battle, owning nothing, praying awfully early in the morning and so on, that there were over 700 different regulations to follow.)[1] 

Though ultimately unsuccessful in their conquest for Jerusalem, the order managed several impressive victories and earned a reputation for military prowess and tight efficiency. And wealth. As the Crusades drew to a close, the organization found itself at loose ends as there were no more battles to fight. Fortunately, a lucrative replacement turned up; international banking. In a sense, the Knights of the Temple would come to serve as history's first ATM machines.

Thanks to more than a century of officially sponsored and religiously sanctioned marauding, the Knights had amassed land holdings, farms, tax rights and most importantly, fortresses scattered through-out the known world. By the end of the 13th century, the order was a notable player in Mediterranean politics and economy. Some of their acquired real estate was conquered, but much of it in mainland Europe came in the form of bequeaths from devout widows and others anxious to donate in the Lord’s name. Where strategically opportune, the Knights built and manned fortresses and often had the rule of the land as the sole local power. Though they did not establish their own sovereign state, the Templar network of forts (‘temples’) ranged from Syria to Cyprus to France and beyond. Along with these strongholds, the order had plenty of knights to man them and safeguard treasures. They even had a navy to rival all but the largest and richest kingdoms during this era of numerous small principalities and post-Crusades lethargy.
Your friendly neighborhood bank. 

And so, enter the free market. Though in the 13th century no economy was exactly laissez faire, European and Mediterranean trade enjoyed a bit of a peace dividend. Silk Road merchants plied their wares from east to west as an end to the Crusades meant a leap forward in global trade. But it was still an age of pirates and thieves, so merchants needed a way to travel safely and, if possible, wire the money ahead rather than risk it in travel. The Knights could, for example, take a merchant’s deposit in Antioch and hand it back, minus a fee, in Marseille. The real money however was not in consumer finance but in central banking. For even larger fees, the Knights of the Temple loaned to kings and guarded in their forts royal treasuries.  In fact, in their main fortress on what is now rue de Temple (Temple Street) in Paris, the Templars safeguarded the dwindling
wealth of France’s King Phillipe. And in 1312, he was having trouble paying off his debts.
Handsome but poor.
So, he did what any medieval monarch would do. He got the Pope’s blessing. Together, Phillipe le Bel (his portraits do show a good looking guy) and Pope Clement V hatched an ingenious and successful plan to get their hands on the Templar booty. First they hired the equivalent of a Washington D.C. public relations firm to bad mouth the Knights of the Temple as heretics and homosexuals. Gossip and pamphlets covered Christendom slandering the order. Then, achieving a remarkable feat of coordination in the days before email and cell phones, the King and Pope arranged dozens of different assaults on Templar forts though-out Europe and the Mediterranean. In order to coordinate such a huge undertaking against such a formidable foe, the royal and papal forces needed a specific date on which to attack. Guess the day.

Amazingly, the plan worked. The Templars were wiped-out. Fortresses were over-run, Knights arrested or killed, ships seized, treasures taken. Jaques de Molay, the last Grand Master of the order was captured and held by Phillipe for two tortuous years after the Friday the 13th round-up. During his imprisonment, de Molay confessed to kissing the Devil’s behind and plotting the overthrow of Christianity and decentness. The coup de grace came in 1314 as the French king ordered the hapless de Molay’s execution. In order to watch, Phillipe had the Grand Master burned on the Isle de Juifs or Jew Island in the Seine which was visible from his corner window in the Conciergerie palace where he resided when in Paris.  (It simply would not do for a king to mingle with the commoners thronging to witness yet another burning on Jew Island.) Today, the spot is a romantic park at the leafy tip of the Île de la Cité in the shade of Pont Neuf.
The site of the 14th century burning
De Molay cursing the powers that be.

Legend holds that while tied to the stake, de Molay cursed the King and the Pope, saying he would see them both before God with-in a year. Sure enough, with-in a year the King died in a hunting ‘accident’ and the Pope was poisoned.

Don’t go picking fights on Friday the 13th.

Other interpretations for the existence of the Friday the 13th myth are thankfully shorter.

For non-Templar Christians, Friday the 13th is unlucky because Jesus was betrayed after the number of people at the Last Supper went to thirteen once Judas, the betrayer, arrived late. 13 became the number of betrayal.  Jesus was then crucified on a Friday. At least in the Middle Ages, few devout Christians considered Friday an auspicious day to get married, open a business or start a trip. They refrained from eating meet on Fridays, and legend claims Louis IX of France, who was so devout he went on to become Saint Louis, even tried to ban laughing on Fridays. 
Thursday the 12th.
In turn, feminists believe the superstition is a product of repression created by Western male-dominated culture. In northern Europe, Friday once belonged to Freya, the Norse mother goddess, as Freya’s Day, while 13 was sacred to women because they have 13 menstrual cycles a year. The moon, often depicted as a female in polytheistic religions, also has 13 cycles in its lunar calendar.

The famous Paleolithic carving of the Venus of Laussel supports this view as she holds a cornucopia with 13 notches in it.

Over time, men have co-opted positive female iconography and belittled it through cultural subversion. Making symbols of motherhood like Friday and the number 13 into a message of evil is therefore part of a long and gradual sexism, which could also include tricks such as using the word ‘mankind’ for all humanity or even establishing impossible role models such as the Virgin Mary or Angelina Jolie.

Though Paris has little to do with Hollywood’s contemporary version of Friday the 13th, the different interpretations for the superstition remain, even if only in moonlight. 

[1] Newman, Sharan. The real History Behind the Templars. Berkley Books: New York. P.51.