When to pick Walnuts for pickling

Hi OBG Not done it myself but have a friend who does theirs in late June-early July ,using the young green nuts before the shell has started to form inside. he checks this with a pin - stick it in at the end where the flower was. If a shell has started to form it will be a quarter-inch or so in, and you will need to discard the fruit. Oh, and watch out for the juice, which will stain skin and some fabrics.