From a food safety standpoint we do not recommend making any recipe with potatoes and dairy a week in advance. Potatoes are a neutral PH and combined with dairy, especially if done warm and not cooled immediately, could lead to some problems. The mashed potatoes will also turn dark from oxidation for that long, even if refrigerated. Instead, make the mashed potatoes the day before, cool, refrigerate covered, then add in the cream cheese (same weights as butter or margarine) the day of your meal.