Dec 4, 2015 - 2:42 PM
The Palais des Nations can be visited by a guided tour from Monday to Friday between 10 am and 12 am and between 2 pm and 4 pm. No exceptions are mentioned for December 31 (a Thursday) or January 1 (a Friday). But to make sure, I suggest you study this information and contact the UNO if necessary. Please see the e-mail address at the page I linked to, under "Reservations". I'm not sure about the sculpture; you can ask the UNO as well.
I hope you're able to find the right information!