Honestly i'm not sure. Official policy is that during peek times either a check in or a check out are required on a Fri-Sun. I'm assuming that Christmas would be considered a peek time.
That said, you may be able to do your 7 night stay by 2 reservations. the first from Mon-sun, and the other for Sun-monday. If the same name is on the reservations (and maybe a mention to the resort to be safe), you could probably combine the reservations onsite so that you can stay in the same room and avoid having to actually do the checkout and leave the room mid-stay.
BUT.... I've personally never tried to do something like that, so I can't be certain how practical or doable it actually is.