I'd second what ninjalad said. If the hotel allows the booking at AMB FWNC rate then you should be fine and only a very "naughty" hotel may try to pull a fast one and mess with one's reservation trying to sell that 31st Dec night.

If OP was concerned one could check with the hotel, but pointing something out to them is what makes them realize that it may not be in their best interest. All depends on the property in the end. I never use AMB FWNC over New Years, but did use it for 23-25 Dec or 24-26 Dec period in the past where the second night for 24-25 Dec or 25-26 Dec rate was significantly more expensive than the first night and had no issues. YMMV.