Normally the weather is pretty nice around christmas time. Usually, it warms up a bit and cools down again mid January, but still unpredictable.
It is supposed to rain out here on Friday and Saturday, usually the stretch of the desert that the mojave trail is on gets pounded like crazy. So prepare, also, the night temps have been in the high 20's on a couple of nights, so dress warm, even though it is 70 degrees in the day. I gets much warmer at night when you get closer to the colorado river. Enjoy, the desert is still a little green from the rain this year, and many springs are still flowing.