Friday and The 13th

Before the 19th century, though the number 13 was considered unlucky, and Friday was considered unlucky, there was no link between them. The first documented mention of a "Friday the 13th" is generally listed as occurring in the early 1900's.
However, documentation aside, many popular stories exist about the origin of the concept:

The Last Supper, with stories that Judas was the thirteenth guest, and that the Crucifixion of Jesus occurred Friday.
That the biblical Eve offered the fruit to Adam on a Friday, and that the slaying of Abel happened on a Friday (though the Bible does not identify the days of the week when these events occurred).
That it started on Friday, October 13, 1307, the date that many Knights Templar were simultaneously arrested in France, by agents of King Philip IV.
However, historically, there is no true date that the Friday the 13th superstition can be linked to.
In the case of Greece, Tuesday, April 13, 1204 was the date that Constantinople was sacked by the crusaders of the fourth crusade. The first ever fall of the richest then Christian city, and the looting that followed, allegedly gave Tuesday 13 its bad meaning. Ironically enough, Constantinople fell for the second time in its history on Tuesday, May 29, 1453, to the Ottoman Turks, a date that puts an end to the Byzantine empire, and to Greek sovereignty for several centuries, and therefore reinforcing Tuesday as an unlucky day in the Greek world.
Many modern stories (including The Da Vinci Code) claim that when King Philip IV had many Templars simultaneously arrested on October 13, 1307, that started the legend of the unlucky Friday the 13th. However, closer examination shows that though the number 13 was indeed considered historically unlucky, the actual association of Friday and 13 seems to be an invention from the early 1900s.
Maybe you're not a paraskevidekatriaphobe. That's the fancy Greek term for people who fear Friday the 13th. Paraskevi is the Greek word for Friday, and dekatria is how Greeks say 13. I know I ain't.