Superstitions abound in the fitness world. They usually evolve accidentally when an athlete has a good -- or sometimes bad -- performance and then attempts to establish cause and effect by reviewing the facts of the performance.
The explanation for 13 being an unlucky number seems to have to do with it being one beyond the perfect 12: hours on the clock, months of the year, apostles of Christ.... Others seem to think it may have something to do with Judas being the 13th -- and last -- to sit at the table.
Up until the patriarchal revolution, both Fridays and 13s were held in the very highest esteem. Both the day and the number were associated with the Great Goddesses, and therefore, regarded as the sacred essence of luck and good fortune.