a two-week's holiday / a two-week holiday

[demir]

Could you kindly explain which of the following uses is true?

I have just returned from a two-week holiday (or, a two- week trip) / a two-week's holiday (or, a two-week's trip.

Thanks in advance.

[Anglika]

A two-week holiday
OR
Two weeks' holiday

I have just returned from a two-week holiday.

I have just returned after two weeks' holiday.

[demir]

Originally Posted by Anglika

A two-week holiday
OR
Two weeks' holiday

I have just returned from a two-week holiday.

I have just returned after two weeks' holiday.

Thank you for your quick reply, Anglika.

I would like to ask one more thing:

If we could say ' a week's holiday' why not ' a two-week's holiday'