The modern reason for this is said to come from Friday October the 13th, 1307. On this date, the Pope of the church in Rome in conjunction with the King of France, carried out a secret death warrant Against The Knights Templar. The Templars were terminated as heretics, never again to hold the power that they had held for so long. There Grand Master, Jacques DeMolay, was arrested and before he was killed, was tortured and crucified.
Friday the 13th was first recorded as being an "unlucky" day in 1869, when the biography of Gioachino Rossini was published.
Considerations from the Knights Templar roundup, Christ being crucified on a Friday, and numerology (triskaidekaphobia - fear of the number 13, etc.) all contribute to the superstition surrounding Friday the 13th.
Personally, I don't believe in luck. "The footsteps of the righteous are directed by God." To me, any Friday, any 13th of any month, the 15th of March (the "Ides of March"), or any combination thereof is just another day to wake up praising God.
Yesterday is gone, and there's nothing we can do about it. Tomorrow is promised to no one. Every day we're given is a gift, that's why it's called "the present."
P.S.: In some cultures, it is Tuesday the 13th or Friday the 17th that's considered unlucky.
On friday the thirteenth 1307 the knights templar were arested, tortured into false confessions and burned at the stake, because the King of France owed them money he could not afford to repay. Friday is also considered unlucky because it was on a friday that Jesus was crucified, but I'm not sure it was the thirteenth.
Those who know about these things, inform us that Adam and Eve were expelled from the Garden of Eden on a Friday, Noah's flood started on a Friday, and Christ was crucified on a Friday. Christians also noted that twelve witches plus one devil are present at Satanic ceremonies so Friday and 13 make a deadly combination.
Friday the 13th was conceived as an unlucky day after what happened to the Templar Knights in 1307. It has nothing to do with witches, adam or eve, and any other mumbo jumbo hoohaa from christianity. It also has nothing to do with Jesus's supposed death on a Friday.