I have no clue, but I would think it would depend on the size of the cake and how long you want to preserve it. We store fondant wrapped in plastic and in air tight containers for it to stay soft. So it makes sense to me if the cake will fit under a plastic cake taker it would stay soft. Or wrapped in plastic wrap if the decorations allow.
The fondant will not get super hard b/c the icing underneath with keep it soft. Also, if you store the cake in the fridge the fondant will remain softer. The condensation that develops when you take it out of the fridge will soften up any hardening it might do.
Well fondant out of its protective wrapping will firm up a bit and it will definitely firm up in the fridge.... but it will come back once it warms up again. If you are just wanting to keep it soft overnight till it is served put it in the box and wrap in in plastic wrap. Its the best you are gonna be able to do 
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