The key things in the cold winter months are water, warmth and wind. Water is incredibly important to ducks, so you must make sure they have a continuous supply of fresh water. When it goes below freezing, regular checks must be made to make sure the surface hasn't frozen preventing the ducks from drinking and ducking their heads.
Warmth: Ducks lose a lot of body heat through their legs! At this time of year make extra sure that they have a good layer of straw in their house.
Whatever the weather your ducks will not want to sit inside moping around all the time! If they don't have a free run of the garden with trees and shrubs to shelter under, make sure that you provide them with something they can sit under and admire the view.