Hi,
The best way to do this would be with a Unity Connection Call Handler, you can easily create a holiday and time schedule for this.
However if you want to use time of day routing in CUCM then you need to do the following;
Create the Hunt Pilot DN in both Time of Day partitions
For the open hours Hunt Pilot will have the Hunt Group associated to it, which is probably what you already have
The closed house Hunt Pilot you will put a call forward on to the external phone number, ensuring the DN has a CSS which can dial that number.
You then will need to add the out of ours partition to all devices so they are able to dial the out of hours hunt pilot.
Guides on completing this can be found on this forum
https://supportforums.cisco.com/document/54501/how-configure-time-day-routing-communications-manager
Hope this helps,
Richard