Finishing a Christmas cake the night before can make the next day easier, but storage depends on what is actually on and inside the cake.
Start with the ingredients
Some frostings, fillings, and toppings require refrigeration while others may be stored differently. Follow the recipe and any food-safety instructions for the ingredients you used. Do not assume every decorated cake can sit at room temperature simply because another cake did.
Protect the decorations
Use a cake carrier, box, or other food-safe cover with enough clearance so the top and sides are not pressed against the decorations. Keep the cake away from strong odors and anything that could bump it.
Think about humidity
Some candies and decorations react to moisture. If a decoration is especially delicate, it may be better to add it shortly before serving rather than storing it on the cake overnight.
For Tomte Cake, separate baking from decorating when helpful
If your family is using The Tomte Cake, you can simplify the schedule by baking ahead and saving the decorating and Tomte reveal for the next day, as long as the baked cakes are stored according to the recipe and ingredients used.
Overnight storage is easiest when you plan it before decorating. Know where the cake will go, how it will be covered, and which decorations are better saved for later.