If you want the BEST time to go, I'd say the second week of Dec, during the week. Avoid the 1st-4th, those days are Candlelight and CM Parties, extra crowds and shorter hours. I would not go after the 14th, as some schools in other parts of the country have that 3rd week of Dec off school, as do most colleges. The best days to go all month would be Wed the 5th, Thur the 6th, and Monday the 10th thru Thursday the 13th. Weekends in Dec are always crowded, but if you have to pick a weekend, I'd pick the 8th/9th.
Now, there likely won't be bad crowds until the 21st, but these are just the best times to go.