Our ancestors had a notion that things get worse as every week wears on. As regards the day one marries, for example, the forecast was "Monday for wealth, Tuesday for health, Wednesday the best day of all; Thursday for crosses, Friday for losses, Saturday no luck at all."
There was a similar pattern if you caught a chill: "Sneeze on Wednesday, you sneeze for a letter; sneeze on Thursday for something better; but sneeze on Friday you sneeze for sorrow." And any feeling of euphoria towards the weekend could be tempting fate: "He that sings on Friday will weep on Sunday."
It is well known, however, that misfortune is cumulative in its effect. When a Friday happens to fall in the 13th of the month, as occurs today, the bad luck associated with the day itself is augmented by the equally male volent aura surrounding the number 13. Today's coincidence is double trouble.
Friday, or Frige Daeg, is named after the Norse goddess Frigg. Frigg had a son called Bandur who was god of light, and the latter was unfortunate enough to be at a banquet in Valhalla with 11 others when his enemy Loki, god of strife, gained entry through a subterfuge.
Once in, this uninvited 13th guest arranged the death of Bandur. Ever since 13 has had a stigma.
In the Christian countries this tradition has continued, partly because of the unlucky significance of the 13 participants at the Last Supper. And Friday, of course, is not only the day associated with the Crucifixion but is also, we are told , the day on which Adam was expelled from Paradise.
The combination of Friday and 13 ought to be no less or no more common than that of any other day and date. If you glance casually at the calendar you will notice that most years have either one or two Fridays on the 13th of a month.
This year, as it happens, we have only one, but some unlucky years have three; this happens in a non-leap year if New Year's Day falls on a Thursday, or in a leap year beginning on a Sunday.
I assume it is leap-year complications which must explain the conclusion reached by experts who have studied a "400-year repeating cycle" of the calendar, and found that the 13th of the month is marginally more likely to fall on Friday than any other day. Each such cycle contains 4,800 13ths these apparently comprise 687 Sundays, 685 Mondays, 685 Tuesdays, 687 Wednesdays, 684 Thursdays and 684 Saturdays.
But there are 688 Friday the 13ths, which seems unfair - or is it just bad luck?