Are you planning to go to a writers conference? Or is it a writers’ conference? Is the Saturday market in the town square a farmers market or a farmers’ market?
This is a construction that often perplexes writers. The first instance in each example is an appositive: a noun phrase consisting of a plural noun that modifies another noun that follows it. The form with the apostrophe is a possessive, a noun that “owns” the noun that follows it.
So if the conference is one that is organized for writers, it’s an appositive. But if it’s a conference organized by writers—one that belongs to them—it’s a possessive. Likewise, if it’s a market for farmers, the proper construction would be the appositive farmers; a market owned by the farmers would be the possessive farmers’.
The trouble with such noun phrases is that they frequently are ambiguous. Lacking insider knowledge, you’re often left to guess whether it’s an appositive or a possessive. Furthermore, there are plenty of commonly accepted constructions that defy appropriate construction.
Children’s Hospital is a case in point. Clearly, the children don’t own the hospital; it’s a hospital for children. But you’ll see the possessive apostrophe on just about every such hospital in the country. One in San Diego seems to be aware of the problem and has hedged its bet. Instead of an apostrophe in its logo, a blue kite with a tail occupies the apostrophe slot. You can choose to read it as an apostrophe or simply view it as a decoration.
An example of an entity that got it right is Publishers Weekly. This is a publication for the publishing industry, not owned by it.
The key is to do your best to determine possession (or not) and punctuate accordingly. So if it’s the boys football team, it’s an appositive. But if it’s the boys’ football uniforms, it’s a possessive.