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I've always understood that the phrase "two weeks" usually turns to "two weeks'" when used as a modifier -- as in "I'm giving my two weeks' notice" or "I get two weeks' vacation" ("two weeks' holiday" for Brits) -- with an apostrophe on the word "weeks", indicating that the vacation belongs to the weeks. One way to explain this is the phrase "two weeks of vacation" being contracted to "two weeks' vacation" -- the vacation is of the weeks, i.e. possessed by it.

But I've seen a lot of people omit the apostrophe in casual writing, and thinking about it, it seems plausible that the noun "vacation" would be plainly modified by the adjectival phrase "two weeks"... but on the other hand, shouldn't adjectival phrases be hypenated, viz. "two-weeks vacation"? But then that seems wrong too; the hyphenate should be singular, "two-week vacation" (like "two-tone shoes") because preceding adjectives are not declined for number.

Why is the singular of “week” used in “two-week business trip”? touches on this (and references a good Wikipedia page on noun adjuncts) but doesn't explain why the plural must be possessive in this case.

Explanation on when the possessive should be used instead of an attributive noun suggests that "two weeks" might be adverbial rather than adjectival, and thus resist being used attributively. But that still doesn't quite explain this particular muddle of pluralization, possession, and attribution...

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Or maybe "two-week vacation". –  GEdgar Feb 12 '13 at 14:30
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3 Answers

up vote 2 down vote accepted

I've always taken the omission here to be or the phrase "worth of". As such, what one means when one says "two weeks' holiday" is actually "two weeks' worth of holiday" and likewise with, for example, notice and imprisonment. The worth in this case belongs to the time, just as the worth belongs to the money when one says "three quid's worth of [insert appropriate noun]".

One can actually then extend this to phrase like "We'll be finished in two weeks' time" which is equivalent to "We'll be finished two weeks from now" but here is like an abbreviation of "We'll be finished in two weeks' worth of time." In all cases, there is some redundancy -- one could simply say "We'll be finished in two weeks." -- but the uses are idiomatic.

Historically speaking, I doubt whether the phrase "worth of" actually ever appeared in using this form of possessive but it may be a helpful way to think about the phrase.

The best explanation to approximate this theory can be found in the Chicago Manual of Style section 7.24. Though not freely available online, you can find it mentioned here where it's claimed the form is inherited from the old genitive case:

Q. Which is correct? (a) He has 15 years’ experience designing software, or (b) He has 15 years experience designing software. I’ve seen it written both ways. I believe “years” needs an apostrophe. If he has 15 years of experience, that would translate to “15 years’ experience.” Right? Please help.

A. You are absolutely right. Analogous to possessives, and formed like them, are certain expressions based on the old genitive case. As your question implies, the genitive here implies of. For some examples, see paragraph 7.24 in CMOS 16.

This line features on the Wikipedia page for genitive case:

Modern English typically does not morphologically mark nouns for a genitive case in order to indicate a genitive construction; instead, it uses either the 's clitic or a preposition (usually of).

So in this argument the possessive/clitic actually stands in for the phrase "of".

This is backed up by one particular line on (unfortunately again) the English possessive Wikipedia page:

The paraphrase with of is often un-idiomatic or ambiguous in these cases.

I should also add that Alex Chaffee's comment in reply to another answer on this question is useful in pointing to the Wikipedia page on the noun adjunct which cites Fowler's Modern English Usage.

For reference, the following sources mention a possessive apostrophe for this form of sentence as the correct use but do not provide any historical background or justification as to this usage:

...

UPDATE

I have just seen another answer providing some examples as possible objections to this usage. In each example, the phrase in question is preceded by an indefinite article. But this would not make sense if one thinks about the phrase I have suggested above.

For example "Take a two-week holiday" is correct; "Take a two-weeks' holiday is not." But that's because one would never say "Take a two-weeks' worth of holiday." Omit the indefinite article and suddenly, intuitively, all seems well: "Take two weeks' holiday."

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Genitive case! Brilliant. –  Alex Chaffee Feb 12 '13 at 19:29
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As @bib has said, two weeks is not possessive in 'two weeks vacation'. A few examples below, if you would consider.

'a ten foot log of wood' [expressing length] – not 'a ten foot's log of wood'

'a three meter high wall' [expressing height] – not 'a three meter's high wall'

'a fifty mile long journey' [expressing distance] – not 'a fifty mile's long journey'

'a six kilo block of stone' [expressing weight] – not 'a six kilo's block of stone'

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Sure, but now try it without the indefinite article, and using the phrase as a direct object, and using a countable noun (not 'wood'). Where "I took a two day holiday" works, you'd need to say "I took two days of holiday", and to remove the of, you'd need to put "two days" in the genitive case (thanks, guypursey) to make "I took two days' holiday". –  Alex Chaffee Feb 12 '13 at 19:27
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There should be no apostrophe. Two weeks is not possessive.

Two weeks vacation is not the same construction as this week's news. The latter describes news that is attributed to a specific thing. In this case, the thing is a particular defined time period. The news in questions is uniquely associate to this week.

The phrase two weeks vacation, however, describes an amount of time, but no particular period. Any two weeks will do. The vacation is not uniquely associated with two weeks. Rather, it is measured by two weeks.

Interestingly, when an indefinite article is used, the plural is omitted

In this job you get a two week vacation but
In this job you get two weeks vacation.

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"Two weeks' notice" (possessive) is most certainly correct (although other constructions may also be correct). "Five years' imprisonment, Three weeks' holiday, etc. Years and weeks may be treated as possessives and given an apostrophe or as adjectival nouns without one. The former is perhaps better, as to conform to what is inevitable in the singular – a year's imprisonment, a fortnight's holiday." - Fowler's Modern English Usage, cited in en.wikipedia.org/wiki/Noun_adjunct -- Interesting point about the indefinite article though. –  Alex Chaffee Feb 12 '13 at 14:51
    
Do you suggest that there is there a logic to using the possessive in two weeks' notice or is it just idiomatic? –  bib Feb 12 '13 at 14:56
    
Well, both, but specifically, what I and Fowler both said: "the vacation is of the weeks, i.e. possessed by it" –  Alex Chaffee Feb 12 '13 at 15:01
    
There is no reason not to write Two weeks notice, five years imprisonment, three weeks holiday and so on. The apostrophe adds nothing. There is a problem when it comes to a holiday lasting one week. One weeks holiday looks strange, because we are not accustomed to seeing one followed by what looks like a plural, but I’m sure we could get used to it. –  Barrie England Feb 12 '13 at 15:17
    
@BarrieEngland I agree and think one week holiday is just fine. In most cases where you say one week, it is preceded by the indefinite article. A one week vacation is standard US English, while a one weeks vacation sound strange. –  bib Feb 12 '13 at 15:22
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