The birthday party is saturday afternoon. Today is Thursday. I have baked and frozen the cake. I just applied the buttercream frosting. Should I wait to apply the fondant pieces, or can/should I put cake back in freezer and wait till sauturday morning? 

No need to refreeze.  If you prefer, you can refrigerate the cake or just leave it (boxed) on the counter.

I agree.  Absolutely no need to refreeze.  And unless you are using a perishable filling then you don't need to refrigerate unless you want to.  I usually do because I find a nice cold cake transport a lot better than a room temp cake.

My typical  timeline when it comes to decorating is fill, stack, frost on Thursday, decorate on Friday, deliver/pick-up Saturday.  Yours is pretty similar so go ahead and decorate and your cake will be perfect for Saturday.

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