It has been my experience that dec/jan weather sort of does what it wants. Sorry i kno this is no solice to you...BUT it has never been really cold
I think 60-65 is probably avg and it gets cooler at night but I am not sure on that. You can find historical weather patterns and yearly avg. on line!
P.S Do not do it last two weeks in Dec IMHO because it gets very crowded and rates start to go up!