Hastings' Dictionary of the Bible (1898)
WEEK. The division of time into portions of seven days found among many different nations which cannot have adopted it from one another - such as the Chinese. Peruvians, etc. - is by some referred back to the order of the creation, and by others to the "seven planets," the principal fact in ancient astronomy. The Jews gave no special names to the days of the week, but simply distinguished them by their number, as the first, second, or third day. The names of the days now in use in the English language are derived from the Saxon, in which they had a mythological signification.
Besides weeks of seven days, which were rendered from one Sabbath to another, the Jews had a week of years, or seven years, and a week of seven times seven years, which brought in the fiftieth or jubilee year.