The number 13, for example:
The Turks so disliked that number that they practically removed it from their vocabulary (Brewer, 1894).
Many cities do not have a 13th Street or a 13th Avenue.
Many buildings don’t have a 13th floor.
And according to information I had a number of years ago, some airlines refused to have a Flight 13 or even a row of seats with that number.
As for the fear of Friday old wives tales say that you should:
Never change your bed on Friday; it will bring bad dreams.
Never start a trip on Friday or you will have misfortune.
If you cut your nails on Friday, you cut them for sorrow.
Ships that set sail on a Friday will have bad luck
You put those two together - Friday and the number 13 - and you can end up with a whole bunch of people that are actually afraid of that date on the calendar. Dr. Donald Dossey, a psycho-therapist specializing in the treatment of phobias estimates that fully 8% (21 million) of the people in America are frightened of Friday 13th
From Jeff Strite’s Sermon: It Was Grace that Taught My Heart to Fear