Friday is named for the goddesses Frigga and Freya harbingers of love, marriage, and prophecy. Fridays were celebrated with weddings and lovemaking and the celebration of the goddess and the crone. Christianity came crashing down and banished the goddesses to the mountaintops declaring them witches. Fridays became days for executions. The bible was writ with tragic events that happened upon a Friday. And a thousand superstitions are born. Don’t cut your nails on a Friday. Don’t get married on a Friday. Don’t visit your doctor on a Friday. Clothing made on a Friday will never fit.
There are thirteen moon cycles within one cycle of the sun, which is why ancient cultures associated the number 13 with femininity. There are 13 witches in a coven. There is no 13th floor, there is no 13th street. The number 13 is almost universally feared in culturally Christian countries.
Friday the 13th was therefore especially sacred. People would stay home and make love all day to honor the goddess of desire. Thus fear and superstition were drawn around that day to keep joy, pleasure, and goddess worship constrained.