Relevant today, I think.

Anyhow... a potted history of Friday the 13th (even more potted, now I've edited it):

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Friday the 13th myth makers  

According to the site, the superstition is derived from myths about both Fridays and the number 13.

Fridays, for example, are hailed as a particularly significant day in the Christian tradition. Obviously, there is Good Friday, the day Jesus Christ was crucified. But according to Christian lore, Adam and Eve also supposedly ate the forbidden fruit on a Friday, the Great Flood started on a Friday, the builders of the Tower of Babel were tongue-tied on a Friday and the Temple of Solomon was destroyed on a Friday...

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...Fridays in the early Norse culture were associated with love and considered a good day for weddings. Over time, however, mythology transformed the Norse fertility goddess into a witch, and Fridays became an unholy Sabbath....

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...In addition to the legendary significance of Fridays, the sixth day of the week also was execution day in ancient Rome and later Hangman's Day in Britain, according the Emery's Web site.

The number 13 also has mythological and religious symbolism.

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. Tradition in both cultures holds that 13 people at a dinner party is bad luck and will end in the death of the party-goers.

Following in that vein, the Last Supper in Christian tradition hosted 13 people and one betrayed Christ, resulting in the crucifixion.

The number 13 also has been associated with death in other cultures. The ancient Egyptians, for example, believed life unfolded in 12 stages, and the 13th stage was death....

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...Finally, Emery suggests the number 13 may have an unlucky connotation because of its association with the lunar calendar (there are 13 lunar cycles in a year) and with femininity (women have 13 menstrual cycles in a year).

Then, there's the event that ties the two superstitions together...

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...It was on Friday, Oct. 13, 1307, that France's King Philip IV had the Knights Templar rounded up for torture and execution. The Knights Templar were an order of warriors within the Roman Catholic Church who banded together to protect Christian travellers visiting Jerusalem in the centuries after the Crusades. The Knights eventually became a rich, powerful and allegedly corrupt order within the church and were executed for heresy.

So, who knows?

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And then, very briefly from this article:

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I just finished reading the abstract of a study published in the British Medical Journal in 1993 entitled "Is Friday the 13th Bad for Your Health?" ... its authors compared the ratio of traffic volume to the number of automobile accidents on two different days, Friday the 6th and Friday the 13th, over a period of years.

Incredibly, they found that in the region sampled, while consistently fewer people chose to drive their cars on Friday the 13th, the number of hospital admissions due to vehicular accidents was significantly higher than on "normal" Fridays.

Their conclusion:

"Friday 13th is unlucky for some. The risk of hospital admission as a result of a transport accident may be increased by as much as 52 percent. Staying at home is recommended."

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