The most common explanation for the belief that Friday the 13th is unlucky is that there was a massacre of the Knights Templar on that date. The Knights formed one of the most important and most well known international organizations in the Middle Ages. Their destruction was one of the most well known events ever to fall on any Friday 13th.
The Knights Templar had been formed in 1118 to protect Christians from Muslims in the Holy Land. They were among the best soldiers of their era. They also ran Europe’s first international banking system. The Knights were good at both fighting and finance. They were bankers with swords.
Christian pilgrims headed for the Holy Land could deposit their funds with the Knights Templar in Europe. The way to Holy Land was dangerous and so the pilgrims were given coded statements of their finance. When they got to their lodgings along the way, their coded statement would be “debited” each time to pay for the accommodation. This early form of credit card saved the need to carry cash.
The Pope and French king were envious as they were too rich and powerful. Also the king needed additional revenue. In those days, there was no regular way to collect tax. He wanted to seize the assets of the Knights (having created all sorts of charges against them). The king’s soldiers moved on them. The pope excommunicated all the Knights in all the lands.
At dawn on Friday October 13, 1307, under the orders King Philip IV of France, a well-planned operation was unleashed to seize every Knight Templar in France. Estimates of the number of knights involved vary from about 2,000 up to 15,000. Only 20 escaped.
Various allegations were made against them dealing with heresy, blasphemy and immorality. Virtually all were tortured to death to make them agree to “confessions”. Many were executed. All of their assets were taken by the king.