Ceremony query

[wolver]

Hello
Can anyone please help me?
Naturalisation visa Online application 20th October 2021 and Biometric appointment booked on 22nd October 2021. I received an email yesterday from my council and they booked my ceremony for 20th December 2021. I still haven't received the home office letter( understand that my application is successful). The issue is I have booked my Internation travel for 2nd January 2022 and returned back 26th January 2022. It's very difficult for me to attend this ceremony and once I had this ceremony I can't go back to my travel ( very important ). I requested my council to change my date to February and they replied just now saying that "The reason why we have booked you in for the 20th December is that your cut out date runs out sometime in January"
Can you please explain to me what is the meaning of the cutout date that runs out sometime in January? I don't understand why they didn't give me a chance to book my ceremony. As per my understanding, I can book my ceremony within 6 months the day I received my letter. Can anyone please help me thanks