In reply to seankenny:
Haha - thanks for the plug Sean!
Actually, you pretty much nailed my suggestions already! J-Tree and Red Rocks are both known as winter destinations. Both fab climbing (RR probably better at the grade for the OP) but I haven't been to either in December so can't offer a personal recommendation.
We have spent Christmas climbing on Mount Lemmon in Tucson, however, and I can certainly recommend that! The climate is as reliable as you're going to get in the States at that time, and there's a tonne of great climbing easily accessed from town.
We've also visited at Thanksgiving (end November) and climbed every day. Here's a few snapshots from that trip.
https://rockaroundtheworld.co.uk/2015/12/05/squeezing-the-lemmon/
https://rockaroundtheworld.co.uk/2015/12/05/pinhead/
https://rockaroundtheworld.co.uk/2015/12/05/the-ruins/
By coincidence we're on our way there now, so check out the blog in a couple of days time for some real-time updates https://rockaroundtheworld.co.uk/ and you can also have a browse through New Mexico which would also offer some sunny December options (but not such a concentration of crags accessed from one centre) Cheers, Dom