Triskaidekaphobia is a fear of the number 13, and annually there is at least one outbreak of triskaidekaphobia as every year has at least one Friday 13th. On that day, about one in ten of the population of the UK will have an intense aversion to walking under ladders, and will try to avoid homicidal chainsaw maniacs wearing hockey masks called Jason1. Architects and builders seem to be prone to it: very few skyscrapers have a 13th floor, often the floor number skips straight to 14 after 12.
But why should the number 13 be singled out as unlucky? Past events have not helped to calm the nervous. One of the most famous examples of the evils of the number 13 was the fateful Apollo 13 flight, which was launched on 13 April at 13:13 local time on Pad 39 (3 times 13). Everyone knows what happened to those guys - they were stranded in space after an oxygen tank explosion and barely limped home with very little power or oxygen left. Another explanation may lie in its arithmetical 'awkwardness': it's a prime number. Or maybe it was because there were 13 people present at the Last Supper?
However, not all cultures have regarded the number 13 as unlucky, In pre-Christian Celtic and Germanic societies it was regarded as important and sacred, and this might also explain why the Christians thought it was unholy (they really didn't get along with Pagans). The Hebrews viewed the 13th year of life as the time to come of age, and it is still celebrated today with Bar Mitzvahs and Bat Mitzvahs. The Mayan and Aztec calendars also featured a favourable 13th period.
And 13 isn't the only unlucky number around: In China the numbers four and seven both sound like the word for death, and in Japan the numbers four2 and nine are also unlucky for similar reasons, they sound like words for death and pain respectively. Numbers ending in seven in Kenya are unlucky, as are all odd numbers in Chad, Nigeria and Benin. The number 786 has a special significance for Muslims and refers to a verse in the Qur'an which roughly translates as 'God be with you'. It isn't surprising then that it is often written as a blessing and it's considered very bad luck to treat this number with disrespect; the only safe way of disposing of it is by fire.
So, 13 isn't all that bad then, you've probably never thought of pain or death every time you wrote down the number nine or number four, nor would you have thought that a 'lucky' seven could have meant something completely different to someone in Kenya. Maybe all those 13s in the Apollo flight were just a coincidence. And anyway, how lucky do you think those men had to be to still be around today in one piece?