This year I won't be making any walnut witches brew, because none of my young walnut trees will have nuts this year. They got caught by the late frost. Two of them didn't even come in to leaf until this month! Even the big old walnut just outside the orchard does not have any nuts forming.
At this time of year I should be thinking about picking the green walnuts for pickling and making into liqueur de noix. Traditionally they are harvested in mid- to late July. You have to catch the fruit before the nutshell inside has got woody. The green fruits are fairly tough all the same, and you have to hack them to bits with a cleaver.
...but after 24 hours the chemical reaction causing the dark colour is well under way.
After three days it looks like you are habouring some sort of nuclear waste product, with the green walnut skins glowing lime green through the British racing green of the liquid.
By Christmas time it can be filtered and put in bottles ready for consumption. The longer you let it mature the smoother it becomes. Depending on your walnuts it will be more or less nutty, often with a touch of bitterness. It goes well on vanilla icecream and I often use it in any recipe I think is robust enough to take it, where the original recipe calls for amaretto.