When people gain excess weight it is because they are sensitive to salt. For them, eating salt/sodium and salty food increases fluid retention in the body, and it is this fluid retention that is the cause of the weight gain. See Sodium in Foods.
Salt sensitivity leads to some fluid retention from alcohol too. So go easy on the alcoholic drinks too.
But the good news is that you don't need to diet, or worry about calories, or avoid fatty foods. The Christmas cake and the cream in your coffee won't make you fat. Just shun the salt.