As I’ve said in answer to a related question, it’s misleading to think of the apostrophe exclusively as a possessive marker. It’s more helpful to think of it as a genitive inflection, certainly capable of expressing possession (John’s car), but also used to specify or classify the reference of a noun (the girl’s face, a bird’s nest or, indeed, the car’s antenna), to indicate time and place (a week’s holiday, the country’s capital) and to refer to a noun that is understood from the context (I’m going to a friend’s (house), Macy’s (store)).

As I’ve said in answer to a related question, it’s misleading to think of the apostrophe exclusively as a possessive marker. It’s more helpful to think of it as a genitive inflection, certainly capable of expressing possession (John’s car), but also used to specify or classify the reference of a noun (the girl’s face, a bird’s nest or, indeed, the car’s antenna), to indicate time and place (a week’s holiday, the country’s capital) and to refer to a noun that is understood from the context (I’m going to a friend’s (house), Macy’s (store)).