Your going to need a telephone tech who knows the key voice system. If you do what I said above and put a new greeting ( a second greeting) in both 800 and 801 then you can do it yourself with the supervisors guide. A tel tech could build a holiday greeting and put dates in that would need to be changed every year.

You may be missing one point; the day night button does not affect what greeting plays, that's based on the times set in key voice, it only affects how the system rings - to phones or to the auto attendant. If you don't have a day night button programmed, you should, the button would over ride the VM automatic time settings.

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