I always recommend frosting and decorating cupcakes with fresh buttercream before serving, because that is when it will have the best texture. Plus, it is usually difficult to freeze them in such a way that the frosting won’t be squished or dented, and the sprinkles may bleed into the frosting when defrosting.  That being said, you can freeze leftover decorated cupcakes for 2-3 months if needed. Thaw them at room temperature before eating. Just know that they may not be elegant Christmas cupcakes anymore