There are many ideas about why this is so, here's what I found when I did a little digging.
Friday it is said is when Eve gave Adam the apple, Jesus was crucified and Cain killed Abel. But look deeper and there are no clear reasons for Fridays to be unlucky (although my grandmother always said, "nothing good ever happened on a Friday" and thrice refused to move into a new home on that day)
And the number thirteen? Judas Iscariot was the 13th guest at the last supper, Loki was the 13th guest in Valhalla when he arranged for Hoor to slay Baldor, and 13 comes after 12 which was a much "better" number in Sumerian culture.
Friday the 13th? On that day in 1307 the Knight's Templar were targeted in a series of pre dawn raids by King Philip of France; Buckingham palace was bombed in 1940, Tupac was killed in 1996 and in 1970 Cyclone Bhola killed 300,000 people in Bangladesh.
Maybe you can see where this is going. Friday is like any day, 13 like any other number. It is our beliefs that "make it" unlucky. As a coach I work with my clients to spot those beliefs, understand what underpins those beliefs and then decide and plan appropriate next steps.