About 100 million people worldwide dread Friday the 13th. Many of them also dread the number 13 in other contexts. The Dictionary of Psychology calls this fear "triskaidekaphobia".

Some triskaidekaphobics take out insurance against the worst that bad luck can dish out on "Black Friday". Others spend the day in bed.

Ships tend to stay in harbour on Friday the 13th. The stock exchange will be comparatively quiet; aeroplane bookings are cancelled; public events may get postponed.

Skeptics, however, know that it's all very silly and often have a good smirk about it. Insurance statistics, for example, reveal no significant difference in accident claims between Friday the 13th and other days.

Most skeptics take Black Friday in their stride. Others take it to extremes. The thirteen members of New York's "Thirteen Club", founded in 1903, hold annual dinners on Friday the 13th on a 13th floor.

Members enter under a ladder, adjust their ties in broken mirrors and take seats under open umbrellas.

The Triskaidekaphobic Society goes to the opposite extreme. They use special diaries in which Friday 13th is missing and replaced by an extra Thursday. They also try to get floor 13 and room 13 renumbered in hotels.

In one USA town the triskaidekaphobics got a law passed to make black cats wear bells on Black Friday so as to be able to avoid crossing their path.