Our system changes during the nighttime hours. How do I change that message?..or remove it?

Not enough information given. What type of system, voice mail?? If it's what I think it is, you will need a tech to straighten it out.

The phone is a Nitsuko 124i. The system is a NEC. It automatically switches to the "Sorry, we're closed..." message at 5:00 & turns off at 8:00 AM. Is there someway to override it?

The answering table schedule in the voice mail is programmed incorrectly. Do you have separate day and night greetings? If no, then you can go into an admin mailbox and override everything. If you do, it's best to get a tech out there and straighten out the schedule.