Why is Thursday the 12th less-unlucky than Friday the 13th?
If I remember correctly, 12 is pretty significant in religions, probably something considered generally holy due to its evenness. 13 however is odd and a bit irregular, likely being linked to more dastardly things. For Friday to be linked to it, I would assume there was some kind of superstition already linked to the day, as many such superstitions based on days exist.
5 bucks says you could figure out a general idea for it on google or wiki with minimal effort.