Paraskevidekatriaphobia
In medieval Europe, Friday was considered an unlucky day to start a journey or any new project, because Jesus was crucified on a Friday.
And the number 13 was considered unlucky because Judas was the 13th guest at the Last Supper. (There's a similar Norse myth where Loki is the 13th dinner guest, and the chaos he brings leads to the death of Baldr.)
Now you know.
And knowing is half the battle. -- G.I. Joe