Week is a division of time which includes seven days. We do not know exactly how this division of time began, but the ancient Hebrews were among the first to use it. The book of Genesis in the Bible says that the world was created in six days and the seventh day, or Sabbath, was a day of rest and worship.
The ancient Egyptians named each day of the week for one of the planets, which they incorrectly believed included the sun and the moon. They considered the seventh day merely as a day of rest and play. Among the later Romans, the days of the week were named after the sun, the moon, and the five planets then known. Each day was considered sacred to the Roman god associated with that planet. The days were known as Sun’s-day, Moon’s-day, Mars’-day, and so on. This system was used about the beginning of the Christian Era. The English names for the days Tuesday, Wednesday, Thursday, and Friday were derived from the names of Norse gods.
See also separate articles in World Book on each day of the week.
