Why is friday the 13th considered unlucky?

I was wondering, why is Friday the 13th considered unlucky? I'm sure there is a good story there but I'm not sure why Friday the 13th is considered bad.

A Friday occurring on the 13th day of any month is considered to be a day of bad luck in English, German and Portuguese-speaking cultures around the globe. Similar superstitions exist in some other traditions. In Greece and Spain, for example, Tuesday the 13th takes the same role. The fear of Friday the 13th is called paraskavedekatriaphobia (a word that is derived from the concatenation of the Greek words Παρασκευή, δεκατρείς, and φοβία, meaning Friday, thirteen, and phobia respectively; alternate spellings include paraskevodekatriaphobia or paraskevidekatriaphobia) or friggatriskaidekaphobia, and is a specialized form of triskaidekaphobia, a phobia (fear) of the number thirteen.

Castro was born on Fri the 13th.

There are a few theories about where the superstition came from.

Before the 19th century, though the number 13 was considered unlucky, and Friday was considered unlucky, there was no link between them.

* 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.

Personally I think Friday the 13th is the luckiest day of the year! Considering I was born on a Friday the 13th. In October none the less.

All I know is that months that begin on a Sunday ALWAYS have a Friday the 13th.

I'm not sure but it might just be a superstition