origin

[P.J]

why is Friday the 13th supposed to be unlucky, where did this come from?

[Quizmonster]

Various reasons for Friday 13th supposedly being unlucky have been suggested. The commonest one is that there were 13 people gathered for the Last Supper in the Bible, which was the occasion - now called Good Friday - when Jesus was betrayed by Judas. However, in pre-Christian times, the Romans already believed the number 13 to be unlucky.
Another is the fact that King Louis IV of France arranged to have the Knights Templar arrested on Friday 13th October 1307 which led to the downfall of that organisation.
A third is that Roman - and later, British - executions were generally carried out on a Friday.
A fourth is the connection with witches in the Middle Ages. Friday was referred to as the Witches' Sabbath and there were supposed to be 13 witches in a coven.
A fifth is that - whilst a baker's dozen meaning 13 was harmless enough - the number was also called the devil's dozen.
It has even been suggested that, because the word �thirteen' comes from �three ten', it is the first number not to have its very own name, as it were, unlike the others up to twelve. That somehow seems to have made it �unlucky'.

[graybags]

King Philip of France and Pope Clement V destroyed the bulk of the Templars (being envious of their power and wealth)
On Friday 13th October 1307.

That is why we say Friday 13th is unlucky - it certainly was if you were a templar

[pigeon toes]

In Spain, It's TUESDAY the 13th that is classed as unlucky., not Friday.

[VS Prasad]

Paraskevidekatriaphobia: Fear of Friday the 13th
http://urbanlegends.about.com/cs/historical/a/ friday_the_13th.htm

A Friday occurring on the 13th day of any month is considered to be a day of bad luck in English, German, Polish, Bulgarian and Portuguese-speaking cultures around the globe. Similar superstitions exist in some other traditions. In Greece or Spain, for example, Tuesday the 13th takes the same role. Friday the 13th is also called "black Friday" in many countries.
http://en.wikipedia.org/wiki/Friday_the_13th

Superstitions swirling around Friday as being lucky or unlucky have existed since ancient times, beginning with the northern nations. Ancient Romans dedicated the sixth day of the week to their beautiful, but vain, goddess Venus.
http://www.coolquiz.com/trivia/explain/docs/fr iday13.asp

Both the Hindus and Vikings reportedly had a myth in which 12 gods were invited to a gathering and Loki, the god of mischief, crashed the party and incited a riot.
http://www.rosslyntemplars.org.uk/friday,_13th .htm