Since when did Friday the 13th become “unlucky” or associated with scary movies for that matter?
The fear of Friday the 13th is called friggatriskaidekaphobia (Frigga being the name of the Norse goddess for whom “Friday” is named and triskaidekaphobia meaning fear of the number thirteen), orparaskevidekatriaphobia a concatenation of the Greek words Paraskeví (Παρασκευή, meaning “Friday”), and dekatreís (δεκατρείς, meaning “thirteen”) attached to phobía (φοβία, from phóbos, φόβος, meaning “fear”). The latter word was derived in 1911 and first appeared in a mainstream source in 1953.
A theory by author Charles Panati, one of the leading authorities on the subject of “Origins” maintains that the superstition can be traced back to ancient myth:
The actual origin of the superstition, though, appears also to be a tale in Norse mythology. Friday is named for Frigga, the free-spirited goddess of love and fertility. When Norse and Germanic tribes converted to Christianity, Frigga was banished in shame to a mountaintop and labeled a witch. It was believed that every Friday, the spiteful goddess convened a meeting with eleven other witches, plus the devil — a gathering of thirteen — and plotted ill turns of fate for the coming week. For many centuries in Scandinavia, Friday was known as “Witches’ Sabbath.”
Another theory about the origin of the superstition traces the event to the arrest of the legendary Knights Templar.
The Knights Templar were a monastic military order founded in Jerusalem in the year 1118. Their original mission was to guide and protect Christian pilgrims along the path from Europe to Jerusalem during the Crusades. Through this mission, the Templars developed a banking system to protect the finances of the traveling pilgrims, then expanded this system throughout their holdings in Europe. Over time, France’s Philip IV of France amassed a debt to the Knights Templar for years of service. He had nearly depleted his money due to his ongoing battles with England. King Phillip became envious of the Knights Templar and their rise to power, so he set his sights on their famed fortunes. Philip devised a plan to arrest all the Knights Templar and charge them with crimes so devastating that no person or group would come to their defense. The charges against them were religious in nature and backed by the papacy of the Vatican and Pope Clement V. His plan had to be swift and carefully put together so as to not alert the Templars in advance.
The connection between the Friday the 13th superstition and the Knights Templar was popularized in the 2003 novel The Da Vinci Code. However, some experts think that it is a relatively recent correlation and is a modern-day invention. For example, records of the superstition are rarely found before the 20th century, when it became extremely common. One author, noting that references are all but nonexistent before 1907 but frequently seen thereafter, has argued that its popularity derives from the publication that year of Thomas W. Lawson‘s popular novel Friday, the Thirteenth, in which an unscrupulousbroker takes advantage of the superstition to create a Wall Street panic on a Friday the 13th.King Phillip’s orders were sent a month in advance to the King’s Men and other Bailiffs. They were not to be opened till dawn on Friday, October 13, 1307. The charges against the Templars were of the highest accusations of heresy: that the Knights Templar asked members to spit on the cross and step on it, to deny Christ, to perform homosexual acts, and so on. The king’s orders were to engage and arrest every Templar in France. All Templar outposts, homes, wineries, mills, and castles were to be taken in the name of the King of France and Pope Clement V. This nationwide arrest was widely successful, and medieval torture tactics were used to obtain confessions from the Knights. This single act against the Templar Order is now viewed as one of the most unlucky days in History – Friday the 13th. King Phillip attempted to further bury the Templars in a public manner: a large event in front of the Notre Dame Cathedral would have Templar Grand Master Jacques De Molay publicly admit guilt of heresy. Instead, the defeated grandmaster took to his forum and apolgized to the people and Templar Knights for his weakness and for signing forced confessions. He then rescinded his original confession and testified to the public that he, his men, and all Templar Knights were innocent, despite their forced confessions. An embarrassed King Phillip was enraged by the old man’s actions and had him burned at the stake along with his second-in-command. De Molay’s dying last words were to curse King Phillip and Pope Clement V, claiming that by the year’s end they both would meet their demise. To add to the superstition of the Friday the 13th and to the power of the Templars both men did die that year. 
In Spanish-speaking countries, instead of Friday, Tuesday the 13th is considered a day of bad luck, commonly referred to as ‘Martes y trece’ (Literally translates to: Tuesday and thirteen). The Fall of Constantinople, when the city fell to the Ottomans, marks the end of the Byzantine Empire. It happened on Tuesday, May 29, 1453. That is why the Greeks also consider Tuesday to be an unlucky day.
According to the Stress Management Center and Phobia Institute in Asheville, North Carolina, an estimated 17 to 21 million people in the United States are affected by a fear of this day. Some people are so paralyzed by fear that they avoid their normal routines in doing business, taking flights or even getting out of bed. “It’s been estimated that [US]$800 or $900 million is lost in business on this day”. Despite this, representatives for both Delta and Continental Airlines say that their airlines do not suffer from any noticeable drop in travel on those Fridays.
There are conflicting studies about the risk of accidents on Friday the 13th. The Dutch Centre for Insurance Statistics (CVS) on June 12, 2008, stated that “fewer accidents and reports of fire and theft occur when the 13th of the month falls on a Friday than on other Fridays, because people are preventatively more careful or just stay home. Statistically speaking, driving is slightly safer on Friday the 13th, at least in the Netherlands; in the last two years, Dutch insurers received reports of an average 7,800 traffic accidents each Friday; but the average figure when the 13th fell on a Friday was just 7,500. However, a 1993 study in the British Medical Journal that compared the ratio of traffic accidents between Friday the 6th and Friday the 13th stated that there is a significant increase in traffic-related accidents on Friday the 13th. There are indications that there are more accidents on Fridays than average weekdays (irrespective of the date) probably because of alcohol consumption. Therefore it is less relevant for this purpose to compare Friday the 13th with any other 13th day of another month.
I suppose it could be scary to think about constantly being in an accident on Friday the 13th. This is possibly the reason for the increase in horror films being released on Friday the 13th.