Encyclopaedia Biblica ( http://en.wikipedia.org/wiki/Encyclopaedia_Biblica )
A Dictionary of The Bible
(Note " (--) " represents Hebrew or Greek letters, which I can't reproduce)
The subdivision of the month into weeks, as also into decades (--)--the week
representing approximately a fourth, the
decade a third, of 29-30 days--is of great antiquity. The old Hebrew for the week of seven days is (--), sabua --i.e., a seven, a heptad (--); cp Gen. 29:27 (--). In later times (--), sabbath, also was currently employed, although only four instances of its use for 'week' are met with in OT--viz., Lev. 23:15 [cp Dt. 16:9] Lev. 25:8 Nu. 28:10 and Is. 66:23--and in Aramaic it became the ordinary word (--). Similarly in NT the week is never called (--), but invariably in (--) or (--) (pl.); cp Mk: 16:9 Lk. 18:12 Mt. 28:1.
This quadripartite division of the month into weeks was naturally suggested by the phases of the moon and was far from being peculiar to the Hebrews. In particular it has been shown to have been an ancient institution with the Babylonians, and even in their case it had nothing to do with the number of the seven planets, after which at a later date the days of the week came to be named. Whether the Israelites used the week as a division of time even in their nomadic stage remains obscure. It is not impossible that they may have derived it from the Babylonians even before their settlement in Canaan, as the Canaanites also had done. However that may be, the development of the seventh day into a day of rest must certainly be referred to the time when the Israelites had already become an agricultural people (see SABBATH).
2. Mode of reckoning.
The mode of reckoning among the Israelites was originally doubtless the same as that of the Babylonians--viz., by dividing the first 28 days of each month into four weeks terminating respectively on the 7th, 14th, 21st, and 28th day, and by making the first week of the new month always begin with the new moon. This intimate connection, however, between the week and the month was soon dissolved (cp the expression 'feast of weeks' in Ex. 34:22 [J]). Whether the preponderance which the Sabbath day, as marking the close of the week acquired over the day of new moon, was a cause or a consequence of the loosening of the connection it is impossible to determine; we are not precluded from supposing that quite other reasons may have contributed to the increased importance attached to the Sabbath; what is certain is that the week soon followed a development of its own, and it became the custom, without paying any regard to the days of the month that did not fit in with the four weeks, to reckon by regular periods of seven days so that new moon no longer coincided invariably with the first day of the week. After this the week of course, having no fixed point of attachment, became quite unsuited as a measure by which the dates of events could be fixed; on the other hand, however, it became useful for the measurement not only of comparatively brief intervals of time but also of periods exceeding a month; thus we not only have the week of marriage festivities (Gen. 29:27 f.), and periods of two weeks (Lev. 12:5) and of three (Dan. 10:2 f.), but also of a space of seven weeks (Dt. 16:9 f. [Ex. 32:22], Lev. 23:15).
3. Specification of days.
When it was desired to specify the precise day of the week on which an event had happened or was expected to happen, the ordinal numbers had to be used as long as the days remained unprovided with special names. Friday and Saturday are the only days that have names of their own; in the OT--if we leave the Apocrypha out of account--Saturday only.
... more on the origins of the names of the days of the week left out. with lots of Hebrew and Greek, some in very small print hard to see, and thats the end of this article.