How many days in advance can you peel potatoes for mashed potatoes
You can do it one day in advance, but keep the potatoes covered with water (and refrigerated).
I'd rather get other stuff done and do the potatoes at the last minute. Do keep them in water whenever making them in advance.
Potatoes lose lots of starch sitting in water. I would do it as close to last minute as possible and always keep fully immersed in water.