The Sabbath or Shabbat was the Judeo/Christian holy day of the week, also known as a day of rest. This day began on Friday at sundown, and lasted until Saturday night. The Muslim faith called this day Friday Prayers. In about 300 CE, most of Christianity changed the occasion to Sunday in order to accommodate Sun worshipers in the Roman Empire. ^{[citation needed]}

TOS episode & novelization: Friday's Child derives its title from a poem first published in Harper's Weekly denoting predetermined characteristics to children born on the various days of the week.