We were there this past December and have absolutely no complaints about the weather. Aulani is on the drier side of the island so that certainly helps, on a couple of excursions we were over on the other side of the island and it was more cloudy but still warm out. The few times that it sprinkled the clouds passed over and then it was sunny and warm.
At 7 months out our reservation definitely took some work. I booked 3 open nights and then kept checking back on the website for openings and in the end managed to get 6 nights at Aulani December 23rd - 29th. We did not have enough points to be able to walk a reservation forward like TALB, but I was still happy with what we did get.
We wound up spending the first 4 nights in Waikiki to get all the touristy stuff done and then headed out to Aulani to relax. I do wish I would have added a couple of nights up on the Northshore at Turtle Bay to the end of our trip, my 2 teen sons loved watching the surfers on the big waves, December is prime season up on the Northshore..... oh well there is always next time!