I am doing a cake for my niece's first birthday for this Sunday and my sister wants fresh fruit in the cake. Will this hold up if I am making the cake on Saturday and stacking it and covering it ganache/fondant that day as well?
I would definitely refrigerate it, and bring it out about an hour before you intend to serve it.
My only other suggestion: if the fruit tends to weep or has a very high water content, spread a thin layer of buttercream, jam, or ganache before adding the fruit.
I always put a layer of whipped icing down before I put on fresh fruit so it doesn't make the cake all soggy and plus, whipped icing is yummy with fresh fruit!