Raw potatoes will turn dark if not submerged into a liquid....or slightly precooked.
You can prepare ahead if all the potatoes are "under milk" or partially cooked.
I would partially cook the potatoes, slice and assemble the dish, then cook.....and remember that a really big dish can take a long time to cook or even heat up if it's been refrigerated.
Linda C

Thanks, I think that's what I will do: partially cook the potatoes.

Or, you can make/bake the entire thing Saturday and warm it up Sunday. Our family -- and I personally -- think(s) the dish actually improves overnight. Wouldn't do this with small/shallow version but the 3-4 quart batches I make work wonderfully. Sometimes add a little milk or cream upon re-heat.

Thanks for the idea. I don't think I want to experiment with totally precooking for the party but I will try it for just the family. I have a fear of it tasting like leftovers!