The number 13 alone scares a lot people especially those superstitious. The more they get scared when it is a Friday! Take note also that in tall buildings 14 floors or higher, they would not number 13th floor as 13th floor! Even if they don't number that floor 13, when floors are counted from below it's still the 13th floor, right? Where did all these superstitions come from?
Having asked around and trying to look it up, Friday the 13th superstition points out to, believe it or not, something related to religion. It is said that there are 12 apostles and Jesus is the 13th person when they are all together! Now why would be 13 bad luck if that is Jesus' number? Some say that 13 is only for Jesus, so for ordinary human beings - tough luck!
Linking Friday to 13 becomes more of bad luck, so they say, because Jesus was crucified on a Friday. It was a matter of putting one and one together for the superstitious! I have not checked if that Friday was a 13 and if it were, probably that's reason for Friday the 13th superstition as a bad luck day.