shutdown linux automatically?

Hi all,
I am a tech at a small library. We have been using Windows forever and I am looking to add Linux to the mix. The one thing that I need to do is to find a way to shutdown the computers at (actually just before) closing time. I know that I could add shutdown -h “now” in crontab at the specified time, problem is we need to display a message telling our patrons that the computers will be powering off and to give them time to save anything they may have in progress. I am open to suggestions.
TIA
Dan