week, period of time shorter than the month, commonly seven days. The ancient Egyptians used a 10-day period, as did the French under the short-lived French Revolutionary calendar. In many regions a four-day to eight-day market week is based on the recurrence of market days; the early Romans observed an eight-day market week. This period also corresponds roughly with the moon's quarter phases, which come every seven or eight days. The seven-day week is said to have originated in ancient times in W Asia, probably in Mesopotamia. This is thought to have been a planetary week predicated on the astrological concept of the influence of the planets, which were long erroneously believed to be seven celestial bodies revolving around the earth; these were the sun and moon and five of the bodies recognized today as planets—Mars, Mercury, Jupiter, Venus, and Saturn. The Hebrew week is based chiefly on the religious observance of the Sabbath, which comes every seventh day and is usually associated with the seventh day of creation, when the Lord rested from his labors. The Christian week and the Muslim week were probably derived chiefly from the Hebrew week, although the weekly holy days are different (Hebrew, Sabbath, seventh day; Christian, Sunday, first day; Muslim, Friday, sixth day). The influence of the weeks of Chaldaeans, Christians, and Jews slowly made itself felt in the Roman Empire, and elements of the systems were probably merged. The planetary week was at first preeminent, and the use of planetary names, based on names of pagan deities, continued even after Constantine (c.321) made the Christian week, beginning on Sunday, official in the civil calendar. The Roman names for the days of the week pervaded Western Europe; in most languages the forms are translations from Latin or attempts to assign corresponding names of divinities. The Latin names, their translations, the English equivalents, and their derivations follow: dies solis [sun's day], Sunday; dies lunae [moon's day], Monday [moonday]; dies Martis [Mars' day], Tuesday [Tiw's day]; dies Mercurii [Mercury's day], Wednesday [Woden's day]; dies Jovis [Jove's or Jupiter's day], Thursday [Thor's day]; dies Veneris [Venus' day], Friday [Frigg's day]; and dies Saturni [Saturn's day], Saturday.