Friday is named after Venus (Freya's day), the planet of love and beauty. In astrology, Friday is actually considered a lucky day for romance, creativity, and pleasure.
The number 13 also has positive associations in many cultures. There are 13 moon cycles in a year, and many ancient calendars were based on this lunar cycle.
The superstition about Friday the 13th being unlucky is relatively modern and has more to do with Christian tradition (Last Supper had 13 people, Jesus was crucified on Friday) than astrology.