ou can say a three-week holiday (with an article and a hyphen) or three weeks’ holiday (with no article and an apostrophe). It is also correct to say three weeks holiday (with no apostrophe). The apostrophe is a matter of style. This punctuation is not in the syllabus, so you should not see it as a test item.
See also the question regarding a sentence in the Sec 1 book:
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