Late December is early for anywhere in the US. Actually, pretty much anywhere period. Last year it was abysmal conditions wise. The season previous to that, most of the resorts were nearly 100% open if not 100% open at that time with great conditions. So it will vary by year. Last season was one of the worst ever, the season previous to that was maybe the best ever. This upcoming season will most likely be somewhere between the two.
Nothing is a sure bet, you'll have to roll the dice. Aspen is a great town and you can have a lot of fun there even if the conditions are not great.