A Friday occurring on the 13th day of any month is considered to be a day of “bad luck” in English-speaking (and, to a lesser-degree, Portuguese-speaking) cultures around the globe. Similar superstitions exist in some other traditions – for example, “Tuesday the 13th” takes the same role in Greek and Spanish-speaking cultures.
The fear of Friday the 13th is called “paraskavedekatriaphobia” – a specialized form of “triskaidekaphobia” (a phobia of the number thirteen).
Oddly enough, there is actually empirical evidence to suggest that Friday the 13th is, in some sense, unlucky. Psychologists have found that a rather large portion of the population is especially likely to have accidents or fall ill on Friday the 13th. This has been attributed to such people feeling a heightened state of anxiety on that day (as in the Nocebo Effect).
In addition, an estimated $900 million is lost in business nationwide each Friday the 13th because people will not fly or do business they would normally.