How do I keep my buttercream icing from cracking overnight?

add a few small drops of vinegar to your icing to prevent the cracking

I don't know of any b'cream that cracks unless the cake has been moved. Then it usually is because the base board is not strong enough.
If you post your recipe we might be able to see something and recommend help.

I used the Wilton b'cream recipe: 1 c shortening, 1 tsp van, 1 lb powdered sugar, 2-4 tbsp water. Thanks for the help or any future ideas. I'll make sure I have plenty of support next time too.

I generally use thicker drums for my cakes. I would get a stronger base. Or, you could add a little corn syrup to your recipe.

I suggest you try some of the recipes on this site in the recipe section. There are many to choose from that taste & handle much better.
Adding either corn syrup or piping gel is very helpful. Another thing is adding a tablespoon or so more shortening or butter *and* flavoring. Then, mix it for up to 5 minutes on low speed. No, you will not over mix it. It will be much smoother and easier to work with