The explanation for 13 being an unlucky number seems to have to do with it being one beyond the perfect 12: hours on the clock, months of the year, apostles of Christ.... Others seem to think it may have something to do with Judas being the 13th -- and last -- to sit at the table.
The fear of Friday is skeviphobia, the fear of the number 13 is triskaidekaphobia, and the fear of Friday the 13th is paraskevidekatriaphobia (aka friggatriskaidekaphobia).