Itīs not uncommon for hotels to impose a minimum stay requirement during Christmas/NYE or force guests to buy a package deal including meals, festivities etc.
Two options I can think of:
1) Contact the sales manager or have your TA contact the sales manager
2) Waitlist for the one night, and your request will be forwarded to the sales team
Iīd go with option #1, you may be able to find a compromise (like staying two nights), but I wouldnīt expect too much as some hotels indeed have an occupancy rate close to 100% during the holidays.