"Preaching to the choir" means directing your message to people who already know it/accept it - the implication is that the speaker is failing to address those who don't yet know the message.
We don't have many (if any) company towns anymore in the United States, but back in the past, when, for instance, a lumber company would set up and own entirely a town for its workers, -- see http://www.humboldt.edu/~eel1/exhibit/show/scotia/scotiahp.html -- one could probably safely assume that the inhabitants would understand that "closed on a company holiday" would mean no stores were open on, let's say, the birthday of the company's founder. Even without the company literally owning the town, there might be so many people who were employed by either one business entity (think Hershey, PA) or by various companies in one industry (think of the cities in what is now the rust belt) that certain company-related dates would be well-known.
The library I work in is a distant satellite of a main branch in a city where some huge percentage of the population works for state government. When the main library decided about six months ago to standardize the telphone answering machine messages in all the branches, they unfortunately neglected to think of the branches outside of that quasi-company town.
My branch used to have a message that gave our normal opening days, with the hours, and then a statement that the library would be closed on (day of week/month/day) for the (name) holiday -- the message was changed after each holiday to reflect the next one. In December, the message would also indicate the days between Christmas and New Years that the library was open, since we got so many calls from patrons who thought we might be closed. No more. In true company town/preaching to the choir fashion, the new message has replaced those statements on closure (and the December special open days announcement) with a generic "closed on state and federal holidays." Quick: what's the name and date of a state holiday in March? How many holidays in February? Is one of them the 22nd? What about the day after Thanksgiving? What about the week between Christmas and New Years? What if a holiday is on a Saturday/Sunday -- will the library be closed on Friday/Monday? (Hint: the answers to the Saturday/Sunday questions are not the same, except for one holiday.)
There is a brief message that one can look on the "library's web page" for more information. Not much help if you are driving, and suddenly wonder if the library is open or closed. Not much help if you don't own a computer. Not much of an introduction to reference service in general.