In short: A lot of people are superstitious about many things. This can be so bad in some that almost takes the guise of a phobia. We go through the history of why we fear Friday the thirteenth and what to do about it. As with any irrational fear the best strategy is exposure therapy. If you have a superstition about anything go for it with relish: step on cracks, walk (carefully) under ladders and seek out some black cats. You will see how your anxieties are totally unfounded.
If you have friggatriskaidekaphobia last Friday was a tough day as it is the fear of Friday the Thirteenth. The name comes from Frigga who is the Norse goddess after whom Friday is named and triskaidophobia is the fear of the number thirteen.
This is actually a fairly modern superstition with little evidence it was seen as a day of ill omen before the mid 19th century. Friday has been seen as unlucky for much longer with mention in Chaucer's Canterbury Tales from the end of the 14th century.
Various suggestions are made as to where it comes from perhaps relating to numerology, the day Christ was crucified and the number of people at the Last Supper. This kind of superstition varies across countries with Spanish and Greeks seeing Tuesday as unlucky and Italians being nervous on the 17th day of the month.
Well whatever the day what can you do to fend off 'bad luck'? Many people have their own superstitions to bring good luck or fend off bad. Logically most people know these are nonsense but they find it hard to resist after all better safe than sorry. Well it turns out our old friend behavioural psychology has an explanation. B.F. Skinner published on the very issue in 1948 but on superstitious pigeons not humans.
The pigeons were given food automatically by dispensers at set intervals, Skinner noted that the pigeons started to repeat behaviour they had been doing when the food came. So despite the fact they had no control on when the food was given they would keep nodding they're heads or turning in circles. Due to now repeating these behaviours chance reinforced them as it was increasingly likely the behaviour and the reward would coincide.
You can probably see how humans are susceptible to this kind of thinking and to add to the reinforcement humans display confirmation bias i.e. we remember when the superstition seems to work, but not when it does not. Also, superstitions provide a sense of control in an otherwise chaotic seeming world. This was all the more the case in the pre-scientific age when humans had no idea how apparently random forces of nature worked.
So there you have it, it is nonsense and the best way not to fall prey to these superstitions is to constantly challenge them and see for yourself how your fortune depends on them not at all. So, go ahead, live a little: break some mirrors, drop some salt, don't knock on wood... it's a bit of a thrill.