week (n.)
Old English wucu, wice, etc., from Proto-Germanic *wikon (cognates: Old Norse vika, Old Frisian wike, Middle Dutch weke, Old High German wecha, German woche), probably originally with the sense of "a turning" or "succession" (compare Gothic wikon "in the course of," Old Norse vika "sea-mile," originally "change of oar," Old English wican "yield, give way"), from PIE root *weik- (4) "to bend, wind" (see vicarious). The vowel sound seems to have been uncertain in Old and Middle English and -e-, -i-, -o-, -u-, -y-, and various diphthongs are attested for it.

"Meaning primarily 'change, alteration,' the word may once have denoted some earlier time division, such as the 'change of moon, half month,' ... but there is no positive evidence of this" [Buck]. No evidence of a native Germanic week before contact with the Romans. The seven-day week is ancient, probably originating from the 28-day lunar cycle, divisible into four periods of seven day, at the end of each of which the moon enters a new phase. Reinforced during the spread of Christianity by the ancient Jewish seven-day week.

As a Roman astrological convention it was borrowed by other European peoples; the Germanic tribes substituting their own deities for those of the Romans, without regard to planets. The Coligny calendar suggests a Celtic division of the month into halves; the regular Greek division of the month was into three decades; and the Romans also had a market week of nine days.
Greek planetary names [for the days of the week] ... are attested for the early centuries of our era, but their use was apparently restricted to certain circles; at any rate they never became popular. In Rome, on the other hand, the planetary names became the established popular terms, too strongly intrenched to be displaced by the eccl[esiastical] names, and spreading through most of western Europe. [Buck]
Phrase a week, as in eight days a week recorded by 1540s; see a- (1).