Do you mean Dec. 30, 2017-Jan. 6, 2018? Or Dec. 30, 2018-Jan 6, 2019?
The web site says you can book 499 days in advance, but it doesn't seem to be registering for the 12/30/17-1/6/18 dates. You should be able to book that now, so my guess is that it's either a web site glitch or it's full which I would guess is unlikely. I would call.