What information does iPhone 4S Log-Aggregated 2012-12-12 contain?
It means that that program (the SMS application) was started 9 times on that day.

Whether that corresponds to someone checking the app in response to a message or simply entering the app isn't logged in that file. I don't know offhand if you can reconstruct that level of detail from any of the standard logs on iOS so you may need a forensic analysis of either the database the SMS app uses or the network / server side of things to track down delivery times or even something as detailed as a daily count of SMS messages.