December is always a crap shoot. More often than not it's just okay. 50-80% of any given mountain is open. Usually on the lower end of that number. Every once in awhile you get a season like last year and everything is open by the Holidays. Then you get this year and you're lucky to have 25% of the mountain open by that time. January is generally a good month in Colorado. Snow fall is typically comes in on a regular basis and the powder days start racking up. It's no guarantee, but it's a more solid bet than December.