The weather in Dec. can be iffy!!
We have visited in Dec. three times and for the most part this is what we experienced...
Chilly mornings (45 to 60 degrees)
Pleasant afternoons (65 to 75 degrees)
Cold evenings (40 to 55 degrees)
It can be windy too, which makes it seem colder.
My suggestion is to layer your clothing so that in the morning you are comfortable, but as the day warms up you can take off your layers and continue to be comfy!!
Hope this helps!! (p.s. look for an occasional rainshower)