You would need to reserve 12/26 and 12/27 as nights regardless of resort. They could also be individual nights. There can be no break in nights when booking continuous LLs. If you reserved 12/26 and 12/27 the dates would now perfectly mirror your trip and you could start booking their LLs at the same time as yours for their entire length of stay from 12/26 - 1/1. As
@Sandisw mentioned if they have date-based tickets starting on 12/28 you could only book the LLs for 12/28 and later, but this would start the same time as your booking window on 12/19 since they will be a continuous onsite guest starting 12/26 if you reserve those two extra nights.