We've gone in December twice, and once in November. First December trip 3 years ago was perfect (for me, at least): temps ranged from 65-73. No rain, mild, but it was too cool for me and DH to swim, though the kids did jump in for a short bit. Last year we went in November and it started out 85 degrees and 4 days later the high was 55, low was 39. Wet, cold, windy. Thankfully, that was the day before we left, but the rest of the week (again, this was before Thanksgiving) was bitter and friends who stayed on said it was hard to tour the parks. We went this past December for 6 days, and we had two days where it was in the 80s, two days were it was gloomy and wet and cool, and two days of mild temps. You simply cannot know ahead of time. You must bring rain gear, cold weather gear (including hats and gloves--we needed them at night), and weather for hot days. Lots to pack, but that's December.