See CMoS 7.15–28
When forming the possessive of a proper noun ending with ‘s’, ‘z’ or ‘x’, use the apostrophe but drop the possessive ‘s’. (Deviates from CMoS 7.16.)
Paris’ wide boulevards; Dickens’ novels; Mendez’ supporters
Retain the possessive ‘s’ for proper names that are spelt that way.
St James’s Church (in London), but St James’ Church (in Oodnadatta)
When forming the possessive of a singular noun ending with ‘s’, ‘z’ or ‘x’, use an apostrophe and an ‘s’; drop the possessive ‘s’ for a plural noun.
the hippopotamus’s tooth; hippopotamuses’ teeth
The following are considered attributive nouns (ie adjectives) rather than possessive nouns and so do not take an apostrophe: (Deviates from CMoS 7.25.)
artists colony
farmers market
backpackers hotel
foreigners office
visitors centre (US: visitors center)
kids club
employees union
readers letters
environmentalists association
Note: the above list is by no means exhaustive.
When presenting time as possessive, use the apostrophe as per any normal possessive.
several weeks’ exploration; a full day’s work