Night service is a fixed button, but, you need to configre night service first. Do a "change listed" and that is where you can enter a night service extension to handle all of your listed directory number calls (main number)as well as operator calls.
If you have CO trunks, you can enter a night destination on page 1 of the trunk group form.
The night service button is fixed on the switchboard button 12, which is directly above HOLD.