I am making a lamb-based pinot noir sauce a day ahead of time. The sauce is finished with butter.
Should I finish it when I take it off the stove? or when I reheat it the next day?
Sometimes finishing is referred to as enriching, and from what I've seen, the finishing ingredient tends to be something that breaks down or otherwise changes in cooking, like walnut oil or cognac. The purpose is to keep the flavour of the ingredient without cooking it; finish it when you reheat it.
I don't think either way is bad, but I'd finish it when you reheat it as it will be ... fresher for lack of a better word.