|Any Friday that falls on the 13th day of a month is considered an unlucky day in English speaking countries. Germany, Poland, and Portugal also share the same superstition with the English speaking world.
Interestingly enough, in Spain and Greece the same tradition of the bad day is on any Tuesday the 13th instead.
Prior to the twentieth century people considered Friday an unlucky day of the week, and 13 an unlucky number. However there's hardly any evidence that these two superstitions were in some way connected to each other.
So how did this phobia tradition start? While we don't have the exact answer, the oldest theory says that Jesus was crucified on Friday and was betrayed by his 13th guest - Judas - on the Last Supper. The first mention of Friday the 13th in books dates back to the 1900s.
Greece has this tradition on Tuesday the 13th because on that day in 1204 Constantinople was ruined by the crusaders - the looting of the glorious and prosperous city which was considered a symbol of Christianity in those days.
It is estimated that Friday the 13th costs US businesses over $800 million in lost sales and revenue, simply because many people would not make an important purchase (e.g. car or house) or schedule an important event (wedding, etc.) as they fear bad luck on that "black" Friday.
Many of these cautious people do not consider themselves superficial, yet they do not want the bad Friday to ruin their day, so they act purely on precautionary grounds and not out of phobia.