Never been early Dec as I am in retail I can't go until after I close Xmas eve but we have been 5 times late Dec, I would expect similar.
Really you can't know until you get there, our 1st visit in Dec was so cold, hats, scarves, gloves all day had to de-ice car every morning, very cold but sunny with it. In more recent years we have had better temps, some nice some cold days. Best was Dec 08 lovely just about every day.
We take clothes for every type of weather, hats, longs, winter coats, gloves right thru to shorts & Tshirts.
Can't help with crowds, late Dec are worst of the year.