Actually, the legend of "Friday the 13th" comes from two sources.
First, the Christ source: there were 12 Disciples, and Christ made the 13th. 13 sat at table at the Passover supper (what Christians call the "Last Supper"). This is one reason why some people think it's unlucky to seat 13 at a table to this day, especially at a dinner party. I'm not joking.
Second: Christ died on a Friday. The correspondence of "13" and "Friday" sort of took hold; both seemingly very unlucky dates.
Because the calendar was Julian (the Roman standard) back then rather than Gregorian (what most of the world uses nowadays), the actual date of Christ's death was measured by a completely different system, and only using 10 months rather than 12. There's no way to determine if it would have been a Friday the 13th, without some extensive backward calculations (and even then, they wouldn't have known it). The correspondence was clearly coincidental.
The second reason that Friday the 13th was so fated was in 1307, October, Friday the 13th was the date of the massacre of the Knights Templar by the French ruler Phillipe Le Bel. The Knights Templar were an esoteric Christian order that was formed to protect pilgrims on their way to the Holy Land. Later, they were instrumental in setting up the great cathedrals in France and other parts of Europe; most notably their work can be seen in the Notre Dame and Chartres Cathedrals. Philip the "Slime" wanted their treasure, which was considerable, and so declared them to be demonologists, worshipping a heathen demon named Baphomet (which was clearly not the case). He succeeded in wiping out all but a few of the Knights of the Temple of Jerusalem (their official name). Those who survived went underground and formed the society of Freemasons (the master builders; appropriate for those builders of the great cathedrals of Europe). This order has survived to this day as what we know as the Masons, still an esoteric order.
(And, I was born on September, Friday the 13th, 1963.)
