The number 13 is considered an unlucky number in some countries. Charles Stewart Parnell had an irrational fear of the number thirteen. The end of the Mayan calendar’s 13th Baktun was superstitiously feared as a harbinger of the apocalyptic 2012 phenomenon. Fear of the number 13 has a specifically recognized phobia, Triskaidekaphobia, a word coined in 1911. The superstitious sufferers of triskaidekaphobia try to avoid bad luck by keeping away from anything numbered or labelled thirteen. As a result, companies and manufacturers use another way of numbering or labeling to avoid the number, with hotels and tall buildings being conspicuous examples (thirteenth floor). It’s also considered unlucky to have thirteen guests at a table. Friday the 13th has been considered the unluckiest day of the month.
There is a number of theories behind the cause of the association between thirteen and bad luck, but none of them have been accepted as likely.
In Italy, 13 is also considered a lucky number, although in Campania the expression ‘tredici’ (meaning 13) is said when one considers their luck to have turned for the worse. (http://en.wikipedia.org/wiki/13_(number))