LEGEND HAS IT: Never change your bed on Friday; it will bring bad dreams. If you cut your nails on Friday, you cut them for sorrow. Don't start a trip on Friday or you will encounter misfortune. Ships that set sail on a Friday will have bad luck, as in the tale of H.M.S. Friday. One hundred years ago, the British government sought to quell the longstanding superstition among seamen that setting sail on Fridays was unlucky. A special ship was commissioned and given the name "H.M.S. Friday." They laid her keel on a Friday, launched her on a Friday, selected her crew on a Friday, and hired a man named Jim Friday to be her captain. To top it off, H.M.S. Friday embarked on her maiden voyage on a Friday — and was never seen or heard from again.
Some say Friday's bad reputation goes all the way back to the Garden of Eden. It was on a Friday, supposedly, that Eve tempted Adam with the forbidden fruit. Adam bit, as we all learned in Sunday School, and they were both ejected from Paradise. Tradition also holds that the Great Flood began on a Friday; God tongue-tied the builders of the Tower of Babel on a Friday; the Temple of Solomon was destroyed on a Friday; and, of course, Friday was the day of the week on which Christ was crucified. It is therefore a day of penance for Christians.
In pagan Rome, Friday was execution day (later Hangman's Day in Britain), but in other pre-Christian cultures it was the sabbath, a day of worship, so those who indulged in secular or self-interested activities on that day could not expect to receive blessings from the gods — which may explain the lingering taboo on embarking on journeys or starting important projects on Fridays.
To complicate matters, these pagan associations were not lost on the early Church, which went to great lengths to suppress them. If Friday was a holy day for heathens, the Church fathers felt, it must not be so for Christians — thus it became known in the Middle Ages as the "Witches' Sabbath," and thereby hangs another tale.
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The name "Friday" was derived from a Norse deity worshipped on the sixth day, known either as Frigg (goddess of marriage and fertility), or Freya (goddess of sex and fertility), or both, the two figures having become intertwined in the handing down of myths over time (the etymology of "Friday" has been given both ways). Frigg/Freya corresponded to Venus, the goddess of love of the Romans, who named the sixth day of the week in her honor "dies Veneris."
Friday was actually considered quite lucky by pre-Christian Teutonic peoples, we are told — especially as a day to get married — because of its traditional association with love and fertility. All that changed when Christianity came along. The goddess of the sixth day — most likely Freya in this context, given that the cat was her sacred animal — was recast in post-pagan folklore as a witch, and her day became associated with evil doings.
Various legends developed in that vein, but one is of particular interest: As the story goes, the witches of the north used to observe their sabbath by gathering in a cemetery in the dark of the moon. On one such occasion the Friday goddess, Freya herself, came down from her sanctuary in the mountaintops and appeared before the group, who numbered only 12 at the time, and gave them one of her cats, after which the witches' coven — and, by "tradition," every properly-formed coven since — comprised exactly 13.