Today is Friday and my nieces birthday party is Sunday evening! Can I make the cake and cover it in fondant today and freeze it until sunday? or should i make the cake today but wait to cover it in mmf fondant on sunday?

Fondant is wonderful because it protects the cake from the elements, so it won't dry out or become stale if made a day or so ahead of time. If the filling is nonperishable, you can just leave the fondant covered cake in a cool, dry place (your kitchen counter) until its time for delivery.
If the filling is perishable, you can just pop the cake into the fridge for storage. About an hour or so before delivery, take the cake out and allow it to come to room temperature before touching. There may be some condensation, but if you let it evaporate on its own (i.e. don't try to wipe it off), it will be fine.
I would not advise you to freeze a fondant covered cake.