I read that it was something to do with the Celtic moon calendar, in western tradition, where the 13th day of a moon-month was always dedicated to some fertility goddess (what Friday is -- named after Freya/Frigga in Germanic languages, and after Venus as 'Vendredi' etc. in Latin-based languages), and the mid-early Christians (think the time when Christianity became mainstream and the state religion etc.) went 'Ick!' at its heathen AND Jewish meaning, and declared it was terribly bad luck.-