As a newbie to exchanging and SFX, I have to say I'm really not liking this exchange company. I'm currently looking for a place to go Xmas week (anywhere!) just to use my expiring bonus week and nothing shows up in the online exchange. I know it's a popular week, but absolutely no hits when I select "Any City". I just want the option to put in a timeframe and see the possibilities and then book.
If you mean Christmas of 2018, I think you're too late (it'll be a very slim chance, IMO). Even with RCI/II, IMO.
I've booked many Christmas weeks with SFX, but I put my request in at LEAST 12 months in advance.
BTW, even owning with Hilton, it's almost a no-go. Especially Hawaii because I think all but 1 resort on the Big Island cannot be booked unless you own the specific Christmas week. I've checked 9 months out (the earliest I can book with Hilton) and nothing for Christmas week. The week before, yes (even as of about 3 weeks ago they still had some weeks before Christmas).
However, you'll want to call in, as the online system only shows readily available stuff. Just like RCI (if I go on RCI I don't think I can find any Christmas weeks in their ready inventory at least for Hawaii, Mexico). Would have to setup an OGS (ongoing search) for those, IMO.
Sometimes, SFX can reinstate a bonus week if it's expired, if you make another deposit, but you'd want to call and check on that.
Good luck!