There are several ways to accomplish this, and what makes the most sense will depend upon how you are utilizing the system. Does this separate department have their own mailbox/call handler in Unity Connection? If so, just assign a new schedule with holidays attached to that mailbox/call handler and it will observe your holiday greeting (make sure to record one).
Or, if this department utilizes UCCX to route all its calls you could program into the scipt a holiday check (search for holiday.xml) that could be pre-populated with all the holidays and the appropriate greeting/action if the check returns true.
On the other hand, if you don't use send calls through Unity Connections before ringing the phone it will require a little more work. The first question here would be "Do you want to apply the schedule to all phones in the department or maybe just a departmental hunt group?"
Depending on the answer, you'd have to create a new partition that will hold a copy of either all DN's in the department, or at a minimum the Departmental hunt pilot DN. This partition will have a time schedule assigned to it that would include the holidays in question. You would be using the time periods/time schedule to tell the partition when to activate, and since it would be easier to tell the partition when to close the phones, you would be programming for the closed holidays rather than when its open (a longer list).
Next, create DN's (maybe on a CTI route point) that forward to backup phones or a closed greeting, etc. This new partition would then have to be put in many of your calling search spaces, in front of the partition that holds the real DNs so the tiebreaker would go to the TOD (time of day) sensitive partition instead of the regular partition(s).
lots of options...
