Do I have to blanch my potatoes, to make scalloped potatoes ahead of time?

The short answer I recommend is yes, almost always. If sliced and then placed into a pot of cold water, bring to a boil for 1-2 minutes till the slices have a rubbery consistency, this blanching will help stop the oxidation process. You can then rinse in cold water and store refrigerated till ready to drain and layer at a later time.

If raw, once the potato is cut you can store in the refrigerator, covered with water for 12-24 hours. Be sure to keep submerged in water to prevent slices from turning gray or brown. Add a tablespoon of white wine vinegar or concentrated lemon juice to a gallon of water to help keep the potatoes from turning dark. Drain and layer.

I use a salad spinner to drain the slices and get almost all the excess liquid off each slice.