I have done madeira cakes when doing party ones as they are a bit more sturdy when it comes to slapping butter cream on them and icing. I don't see a problem with making the cake on friday and decorating it saturday though.
When you cover the cake with the buttercream, I would place it in the fridge for about an hour before putting the icing on it, it will make it firmer and easier for you to put the icing on (I assume you mean fondant icing?).
When I did my fist cake, I made the mistake of putting it into a tuppaware box with a lid on - this made it sweat, so all I do now is put a clean tea towel over the cake to keep any dust/flies off it.