I need to have two cakes done by this Saturday. One is a 1/2 sheet cake and the other is a 6" round with 3 layers, both with buttercream and fondant toppers. When should I start making the actual cake and icing them? How long can I keep them chilled in the fridge? 

Hi Monique, in this situation I bake wednesday afternoon(because I work during morning). Thursday afternoon I am making fill and putting in cake and around cake. Resting in cooler over night. Friday is for finishing cake, fondant and decoration. Again resting and cooling over night. Dont worry. Cake will not be old. Cake is like vino, needs time to rest and bloom. :-)

I prefer freezing to refrigerating because it's my experience freezing actually helps the cake, refrigerating does not.  I bake early in the week, depending on how busy the week is I will either wrap & freeze or tort, fill, crumb coat, wrap and freeze.  I take the cakes out by Thursday night to sit in the fridge and do the final frost and decorating on Friday.

What Pastrybaglady said and I would make the fondant toppers now. They need time to firm up and I always make extras, as a back up .......in case one gets broken , better safe than sorry.

Thank you all for the feedback!!! :)

Yes,I agree w/pastrybaglady.   the sooner you bake the less rushed you will be when you need time to decorate those cakes :)   even make up your b'cream and fondant anytime you want along w/making those toppers.  

Thank you very much kakeladi!! :) I appreciate your input :)