Now somewhere (& I can't for the life of me remember where) I have read that the F13 notion is quite recent. Whilst Friday has often been considered an unlucky day (especially, a bad day for ships to set sail), the Norse, for example, considered it a lucky day to get married.

As for thirteen, in medieval times it was considered lucky to set a hen to sit on 13 eggs, so that's hardly unequivocal either! The junction of the two 'unlucky' dates, as far as I recall, was stitched together around the turn of the 19th/20thC. But I'll be damned if I can remember where I read it (though it wasn't in Brewer's)!