begin quote from wikipedia under the heading Friday the 13th:
The fear of Friday the 13th is called friggatriskaidekaphobia, frigga meaning "Friday" and triskaidekaphobia (or paraskevidekatriaphobia) being a word derived from the concatenation of the Greek words Paraskeví (Παρασκευή, meaning "Friday"), and dekatreís (δεκατρείς, meaning "thirteen") attached to phobía (φοβία, from phóbos, φόβος, meaning "fear"). The word was derived in 1911 and first appeared in a mainstream source in 1953.
HistoryAccording to folklorists, there is no written evidence for a "Friday the 13th" superstition before the 19th century. The earliest known documented reference in English occurs in an 1869 biography of Gioachino Rossini:
[Rossini] was surrounded to the last by admiring and affectionate friends; and if it be true that, like so many other Italians, he regarded Friday as an unlucky day, and thirteen as an unlucky number, it is remarkable that on Friday, the 13th of November, he died.end quote.
Another story I heard was in regard to the arrests, torture and execution of hundreds of Knights Templar by King Phillip of France in 1307: begin quote from wikipedia heading "King Phillip of France" begin quote.
Suppression of the Knights TemplarPhilip was hugely in debt to the Knights Templar, a monastic military order who had been acting as bankers for some two hundred years. As the popularity of the Crusades had decreased, support for the Order had waned, and Philip used a disgruntled complaint against the Order as an excuse to disband the entire organisation, so as to free himself from his debts. On Friday, 13 October 1307, hundreds of Knights Templar in France were simultaneously arrested by agents of Philip the Fair, to be later tortured into admitting heresy in the Order. The Knights Templar were supposedly answerable only to the Pope, but Philip used his influence over Clement V, who was largely his pawn, to disband the organization. Pope Clement did attempt to hold proper trials, but Philip used the previously forced confessions to have many Templars burned at the stake before they could mount a proper defence.
In 1314, Philip had the last Master of the Templars, Jacques de Molay burned at the stake. The account goes as follows: "The cardinals dallied with their duty until March 1314, (exact day is disputed by scholars) when, on a scaffold in front of Notre Dame, Jacques de Molay, Templar Grand Master, Geoffroi de Charney, Master of Normandy, Ilugues de Peraud, Visitor of France, and Godefroi de Gonneville, Master of Aquitaine, were brought forth from the jail in which for nearly seven years they had lain, to receive the sentence agreed upon by the cardinals, in conjunction with the Archbishop of Sens and some other prelates whom they had called in. Considering the offences which the culprits had confessed and confirmed, the penance imposed was in accordance with rule—that of perpetual imprisonment. The affair was supposed to be concluded when, to the dismay of the prelates and wonderment of the assembled crowd, de Molay and Geoffroi de Charney arose. They had been guilty, they said, not of the crimes imputed to them, but of basely betraying their Order to save their own lives. It was pure and holy; the charges were fictitious and the confessions false. Hastily the cardinals delivered them to the Prevot of Paris, and retired to deliberate on this unexpected contingency, but they were saved all trouble. 'When the news was carried to Philippe he was furious. A short consultation with his council only was required. The canons pronounced that a relapsed heretic was to be burned without a hearing; the facts were notorious and no formal judgment by the papal commission need be waited for. That same day, by sunset, a pile was erected on a small island in the Seine, the Isle des Juifs, near the palace garden. There de Molay and de Charney were slowly burned to death, refusing all offers of pardon for retraction, and bearing their torment with a composure which won for them the reputation of martyrs among the people, who reverently collected their ashes as relics'.
So, take your pick for there are many reasons down through history for people to be superstitious about this number.
However, if you consult numerology the number 13 is actually a power number, as in the 13 colonies of the United States and to Christians as the 12 apostles and Christ which made 13. So, whether you are scared by power or not (whether good or bad) 13 does indeed appear to be a power number.