I am making a cake covered in fondant with fondant topper for Saturday, wanting to know how soon to cover and steam... not sure if it's okay to cover and leave in the fridge overnight then steam it Saturday morning??

I am assuming you want to steam the fondant so it gets shiny.    If you take it out of the fridge it will probably sweat.  I think you should probably need to let the fondant get dried out before you steam it.