They spent three days’ vacation building terraces and landscaping the yard!
Explanation:
The sentence <em>"they spent three </em><em>days’</em><em> vacation building terraces and landscaping the yard!"</em> has a possessive plural noun. When there is a plural noun that ends with a final "s" the apostrophe must be placed after the "s" of the plural noun. It is not correct to say<em> "They spent three </em><em>day's</em><em> vacation" </em>because It is not a day or one day in its singular form. It's been three days.
