Can you make mashed potatoes a day ahead of time?

You can, but they won't taste as good.

You can cook the potatoes a day ahead and then heat them in the microwave and add hot milk or cream when you're ready to serve them. If you do this, I wouldn't MASH them until they've been reheated.

Yes you can when you use cream cheese & sour cream instead of milk. They are called Make Ahead Mashed Potatoes.....I use garlic powder instead of onion powder.