A seven-day stretch with an older name
In ordinary English a week is any run of seven days, or the calendar block that starts on Sunday in much of American usage and often on Monday in Britain. People also use it loosely for the five-day working stretch, and in a few set phrases for a date seven days away: Thursday week is not this Thursday but the one after. The noun is native Germanic English, not a Latin or French loan. The calendar idea of seven named days is younger in northern Europe than the word that came to label it.
That mismatch is the whole story in miniature. Speakers needed a name for a repeating slot of time; they already had a noun that meant a turn or a place in a series. Once the seven-day cycle spread north, that noun was the one that stuck.
Native English, not a borrowing
The modern spelling settles in the sixteenth century from Middle English weke, wyke, and a crowd of other vowels. Those forms continue Old English wice, wicu, and wucu, a weak feminine noun already used for a seven-day span in late Anglo-Saxon writing. School texts of the period gloss it with Latin ebdomada. The line of descent is inheritance, not borrowing: Proto-West Germanic *wikā, from Proto-Germanic *wikǭ, also reconstructed as *wikōn-. Close kin include Old Frisian wike, Dutch week, Old High German wehha and wohha, modern German Woche, Old Norse vika, Swedish vecka, Danish uge, and Gothic wikō.
Old and Middle English never agreed on the vowel. After initial w, dialects produced e, i, o, u, y, and mixed spellings. One northern line, from Old English wucu, became Middle English woke and wouke. In Scots the w often dropped before the rounded vowel, leaving ouk, oulk, and related spellings whose written l was never sounded. Standard English week continues the weke type rather than the woke type. The diversity is a phonetic effect of w on the following vowel, not a sign of several unrelated words.
A turn, then a measure of days
The Germanic noun is usually taken to have meant a turning, a succession, or a change before it meant seven days. Gothic uses wikō once, in the Gospel of Luke, to render a Greek word for an ordered course of priestly service, not a calendar week. Old Norse vika could mean a sea-mile, originally the stretch of a voyage between a change of oars. Old English had the related verb wican, to yield or give way. English weak belongs to the same family of bending and giving way, as does dialect wick in the sense of a corner.
Deeper reconstruction is not settled. One standard account traces the cluster to a Proto-Indo-European root *weik- meaning to bend, wind, or turn, with the time sense growing from the idea of a recurring period. Another view, associated with Dirk Boutkan, notes that a nearby root meaning exchange would also fit a returning cycle. Latin vicis, a turn or alteration, is often compared, as are verbs of yielding in Greek. None of this is a tidy folk story about the moon. Carl Darling Buck recorded the guess that the word once named a lunar half-month or some other older division, then added that there is no positive evidence for that step. Where the evidence stops, the article should stop.
The morphology is simple: a native weak feminine noun, with no prefix and no suffix that still does independent work in Modern English. What looks like complexity is dialect spelling, not hidden structure.
The seven-day cycle arrived from outside
Germanic speakers almost certainly did not count time in seven-day weeks before they met Roman practice. Historical dictionaries make that point with some force: there is no reason to suppose a native Germanic week in the calendrical sense. The seven-day week itself is ancient, often linked to dividing a roughly twenty-eight-day lunar cycle into four phases, and it was already the Jewish week later carried through Christian Europe. Rome popularized planetary day-names. Germanic peoples kept the seven-slot frame but swapped in their own gods, without matching those names to planets.
Other European systems had divided time differently. Greeks more often split the month into ten-day stretches. Rome also had a nine-day market cycle. A Celtic calendar from Coligny points to half-months rather than weeks. So the inherited noun was ready for a new job. In West Germanic and Scandinavian it became the ordinary name for the imported seven-day unit, while Romance languages built their usual words from seven or from the sabbath week.
Later English uses stay close to that seven-day core rather than inventing a new root sense. By the late fourteenth century, week of years could mean seven years. From the 1540s, a week appears in phrases such as eight days a week. The compound weekend is later still, as are weeknight and the verb sense of spending a weekend away. Colloquial knock someone into next week is nineteenth-century pugilism. Those are extensions of counting, not returns to the old idea of a turn in a series.
What the history leaves in plain sight
English did not calque Latin septimana or Greek hebdomas, though both were known to scholars and both survive in learned English as septimana and hebdomad. The everyday noun is a native label whose first sense was closer to a turn in a series than to Sunday-through-Saturday. German Woche and Dutch week still show that shared inheritance. Scots ouk is the same word with the initial w lost. The homophone with weak is not an accident of later spelling: both sit on the old idea of bending or yielding.
One concrete remnant is the Gothic citation. In the only surviving use, wikō is a course of duty, a slot in an ordered rotation. That is the older meaning still visible behind the calendar. A week in Modern English is a turn that comes around, even if we now count it strictly as seven days.