A tip when making icing/frosting as in butter cream, royal etc is to make it so that when its mixed, it will hold its shape. ie.. if when you stop mixing it all just forms back into one shape with no edges, you need to add more icing sugar. (i hope that makes sense). You can definately make it the night before and leave it in the fridge.. just bring it out for a little while before you use it otherwise you may find it a little too stiff.

Best of luck