How long please would a cake that is dirty iced with smbc stay fresh .

I wanted to make it tomorrow for Saturday?

Thank you

If it was me I would crumb coat, chill for about 15 minutes to let it set, and then final ice the cake.  Then I would put it into a cake box and keep it in the fridge.  The only thing would be to make sure you don't have anything that has a strong odor in the same fridge because it could transfer the smell/taste into the cake.

But the cake should be fine if kept chilled for Saturday.

Agree with Freckles. I've left SMBC covered cakes in the fridge for a few days and they stayed fresh. 

I'll just add that you might want to bring it out of the fridge ahead of time on Saturday to let it come back up to room temperature. SMBC loses that silky smooth texture when the butter is still cold right out of the fridge. 

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