Week
2 of the 7 encyclopedias on this shelf carry an entry for Week. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
the space of seven days; the space from one Sunday, Monday, etc., to anoth another; the most obvious and convenient division of the natural or lunar month. The division of time into weeks did not exist among the aborigines of America when the New World was discovered, nor did it exist among the Polynesians, the Japanese, or, it is now believed, the Chinese. It is nearly universal in India, and was found thoroughly rooted when the first Christians went to that country. So has it been from a period of high antiquity in Scandinavia, the names of the several days being connected with identically the same planets in the two regions; so that, if at noon on Sunday in Sweden one could be transin a moment to India, he would ported ported find it Aditwar (Sunday) y) there, and so of any other day in the week. The Hebrews, and it is thought the other Semites, had the institution of weeks, the days apparently being simply numbered first, second, third, etc. During the early centuries of their history the Greeks and the Romans had not the institution of weeks, there having been ancient forgery in connection with Homer's oft-quoted passages on the subject. Dion Cassius, S century after Christ, considthat the week with the planetary names of the days had been introduced into Rome only recently, and from Egypt. The establishment of Christianity under Constantine confirmed the change, and thence the septenary division of time spread to the whole Christian world. The Mohammedans borrowed the week from the Jews, and like them number the days, as do also the Greeks, Slavs, Finns, and as did the French revolutionists, instead of naming them Teutons, and Celts. like the Latins, Teutons, One school of theologians attributes the wide prevalence of septenary institutions to the Sabbath having been divinely instituted at the Creation; another school again regards the week as a fourth part of a lunar month. Also applied to the week days as opposed to Sundays.
Aiton's Encyclopedia (1910)
a period of seven days. Although it is one of the commonest divisions of time, it is of unknown origin. The Greeks and Romans divided the month into three periods each. Although the people of India have names for the successive days, the week is said to be unknown. The week was introduced into modern Europe by the Christians, who, in turn, derived it from the Hebrews. The week is not an exact division of either the month or the year. It appears to be based rather on the Hebrew account of the origin of the world, according to which the Creator worked six days and rested the seventh. See Sunday ; Monday, etc.