In the Christian tradition, Friday - any Friday - is associated with the Crucifixion, and therefore viewed with circumspection as a day of possible ill-omen.
This suspicion is compounded by the fact that many fateful biblical events took place that day: Eve allegedly gave the apple to Adam on a Friday morning, and that very afternoon the ill-fated pair were evicted from their Paradise.
Moreover, Noah's Flood began on Friday, and it was also the day the Temple of Solomon in Jerusalem was destroyed.
But if any Friday is unlucky - and Good Friday, almost by definition, must be more than most - a Good Friday on the 13th must surely be a day when Murphy's Law becomes a force majeure; surely today, as Hamlet puts it, "when sorrows come, they come not as single spies, but in battalions", or as Edward Young perceived the situation:
Woes cluster; rare are solitary woes;
They love a train, they tread each other's heels.
The significance of 13 in this cluster of accumulating woes relates to the history of the number 12, seen as the most "complete" number. There were, after all, 12 months of the year, 12 gods of Olympus, 12 signs of the zodiac and 12 apostles of Jesus. Thirteen, just one digit beyond 12, was symbolic of the first departure from completeness, or an initial step towards evil. Judas Iscariot, for example, was the "13th" apostle, and the 13th tribe of Israel was the only tribe to be left without a tract of land.
In Norse mythology the number 13 is associated with the death of the god Bandur. Bandur, god of light - and coincidentally the son of the goddess Frigg after whom Frige Daeg, or "Friday", is named, was at a banquet in Valhalla with 11 of his friends when Loki, god of strife, gained entry through a subterfuge. Once in, this 13th guest, it seems, arranged the death of Bandur.
It is because of the association with Frigg that one who suffers from a morbid fear of Friday 13th has come to be called a friggatriskaidek aphobe. Some afflicted with this malady have apparently found that communal aversion therapy works a treat; the Friday the Thirteenth Club, for example, exists for the sole purpose of dispelling any notion that ill-luck attends days like today, and whenever one comes along, enthusiastic members hold a party to dance under ladders, shatter mirrors, and prance around indoors with umbrellas.
The only alternative for friggatriskaidekaphobes on Good Friday the 13th is to stay in bed all day.