13 is the number of Archimedian solids.
The fear of the number 13 is called Triskaidekaphobia. A specific fear of Friday the 13th is called Paraskavedekatriaphobia or Friggatriskaidekaphobia.
There is always
at least one Friday 13th in each year.
According to historians, there were 13 people at Christ's Last Supper and Christ was crucified on Friday 13th. So, Friday 13th is considered as unlucky... But thirteen may be considered a 'bad' number simply because it is one more than 12, which is a popularly used number in many cultures (due to it being a highly composite number). When a group of 13 objects is divided into two, three, four or six equal groups, there is always one object leftover.