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a two-day holiday or a two days' holiday? which one is right? or are they both right?

Different dictionaries have different ideas about it, so i feel very puzzled. can any native English speaker help me? thanks!
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These are correct:

a two-day holiday
two days' holiday

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Comments  
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Once "a" is introduced it becomes singular, thus a two day holiday is right

train grape 794a two day holiday is right

The hyphen is not optional: "a two-day holiday". Also, every written English sentence ends with a mark of punctuation; there are no exceptions.