The tours may be filling up towards the end of the year because of additional Fremont deliveries. It's convenient to take a factory tour in conjunction with delivery of your car from the factory. And pushing year/quarter-end Fremont deliveries (at the expense of overseas or cross-continental deliveries) help Tesla meet their short term delivery goals by eliminating transit time.
What if Tesla is purposely blocking out tour slots during December to leave spots open for Model-X owners, in case they want to pick up from factory and book a tour? That would mean some some slots could come free and putting your name on a waiting list might be fruitful.