The number 13 is widely considered unlucky, so much so that certain buildings do not list a 13th floor. It’s hard to avoid 13 when it makes up a day each and every month, and even more so when the 13th falls on a Friday.Every year people are treated to at least one Friday the 13th on the Gregorian calendar, but some years there can be as many as three such days. It is difficult to pinp...