Expect 2 hours of daylight, up to -40 degrees centigrade temps and shit loads of snow. Also don’t expect many other tourists at that time, although that could be a blessing.
 
And Oswiecim is only a ‘must’ for fucked-in-the-head death tourists & freedom haters.
 
 

I love how Heavydrinker makes all these asinine remarks, then his sig says “be easy.”
FWIW, I seem to remember it getting dark around 4 or 4:30 in the UK in late December.
This site gives you an overview of weather and temps for northern Europe during 2006-7. link

asinine? Your 10 cent words have no place in this 5 cent conversation

You will definately want to pack thermals as you might go an entire week of below freezing weather.  Snow doesnt get too deep, but its aggravating as hell to walk in (especially for those of us who are not used to it). 
Of course the amount of daylight depends on how far north or south you go, I think I remember having about 7 hours of good daylight in Prague in Jan. 
 

For those of you from “the bridge may ice in cold weather” regions.

Good insulated footwear is more important then thermals when you plan to spend a lot of time outdoors when it’s cold. (“Cold” is a personal thing, but “above freezing” is not cold.) It is easy to warm up your body by moving actively, but it’s nearly impossible to warm up your feet this way.

Even if you disagree, at least leave your flip-flops at home!