Number 13 Is Known As The World's Unlucky Number. What Is The Reason?

It is indeed true that many people, all across the world, and most especially in the Western part of the world, and in Europe, consider the number 13 to be an extremely unlucky number.
Most people prefer not to sign contracts on this day, will not open a new shop on this day, and most building will not have a thirteenth floor!
The reason for all this superstition is that when Jesus was crucified, and at his Last Supper, there were thirteen people at the table with him. Furthermore, since there were twelve tribes in Israel, the thirteenth one was considered unlucky!
According to Norse mythology too, thirteen is a lucky number. It seems that Loki, one of the most evil of the Norse Gods, went to a party that was meant for twelve people, thus bringing bad luck to everyone!
Whatever it is, even today, most people refrain from inviting thirteen people for dinner!