I've been in December/January time frame a few times and the weather has been different every time. One year it was in the 70's and 60's so we barely needed jackets (and we'd brought matching Christmas Disney hats and gloves and mittens so were disappointed not to be able to use them). One year it was in the 50's and 60's, for me it was heavy jacket - like fleece lined wind breaker, but not super warm winter coat weather. Last year, it was COLD. Colder than it was back home. Record lows. Winter coats, gloves, scarves, hats, thick socks, etc.
The tip to have layers is a good one. You will *probably* be fine in pants the whole time, although you never know...maybe pack a pair of shorts too just in case. Then have a t-shirt, sweatshirt, jacket and heavy winter coat with you. It is better to bring the winter gear and not need it, then need it and not have it.