In general, most Japanese don't often send cards for Christmas, but when they do, this is usually what they say. It is more customary to send postcards to ring in the new year, in which case, you would say something like "あけましておめでとうございます．今年もよろしくおねがいします。" This means something to the effect of "congratulations on the new year, I look forward to (your friendship/acquaintance) this year as well." Hope this helps.