Structure the DAY message to say "Thank you for calling XYZ Company. Our normal business hours are 8am to 5pm, Mon. thru Fri. We are closed daily from Noon to 1pm for lunch. If you are receiving this messsge during normal business hours, we are assisting other clients. If you know your party's extension number......."
Set up your NIGHT message as normal.
Set the Auto Attendant to follow the weekly business schedule, make it answer delayed with the Night Service making it answer immediately.
The Night Service button will determine how many rings before answering, and the Business Schedule will determine which message plays.