Week (Anglo-Sax. Weoc), a period of seven days, a division of time adopted by the ancient Egyptians and Hebrews, and in general use among Christians and Mohammedans. Its origin is referred back in one part of the Mosaic account (Exod. xx. 11) to the creation of the world, in another (Deut. v. 15) to the exodus from Egypt. Josephus, Philo Judaeus, Clement of Alexandria, and others, speak of the week as not of Hebrew origin, but common to all the oriental nations. It was not in use by the Greeks and Romans, until adopted by the latter at the period of the introduction of Christianity, after the reign of Theodosius. Its adoption was no doubt hastened by the peculiar convenience of such a division of the lunar month into four parts, and by its being so nearly an aliquot part of the solar year of 365 days. The only explanation of the origin of the names given to the days is that by Dion Cassius in his Roman history (1. xxxvii., c. 18, 19). They were founded, he says, upon the names of the seven planets known to the ancient Egyptian astronomers, which they arranged as follows in the order of their distances from the earth, beginning with the most distant: Saturn, Jupiter, Mars, the sun, Venus, Mercury, and the moon.

According to the ancient astrology, each of these planets presided in turn over the successive hours of the day, and each day was named from the planet to which its first hour was dedicated. Beginning with Saturn, on the first hour of the first day, and allotting to each hour a planet in the order named, the first hour of the second day, it is found, would fall to the sun, of the third day to the moon, of the fourth to Mars, of the fifth to Mercury, of the sixth to Jupiter, and of the seventh to Venus. The Latins adopted these designations in their names of the days of the week, as dies Saturni, dies Solis, dies Lunoe, etc.; and modern nations have retained the same terms, those speaking languages of the Teutonic stock substituting in some cases the names of their own divinities for the corresponding ones of Roman mythology. In the ancient Brahmanical astronomy, the week is also a recognized division of time, and the names of the days are from the same planets and in the same order as those in use by the ancient Egyptians; but the week began with them with Sukravdra, the day of Venus or Friday. The Egyptian week began, according to Dion Cassius, on Saturday. This day was also the sabbath of the Jews. Each day being ruled by its particular planet, astrologers assigned to each a particular character; and they may naturally have regarded the day ruled over by Saturn, the most sluggish and gloomy-looking of the planets, as at once a day of rest and a dies infaustus, when all work would be unfortunate.

The Chinese and Thibetans have a week of five days, named after the five elements, iron, wood, water, feathers, and earth.