They fall off the tree when ripe.

True, fruitnut; but some trees drop a lot of immature (unpollenated?) nuts early- just noticed a bunch underneath a woods' edge tree at the north end of the farm this weekend. Somewhat smaller than the full-developed ones will be when they're ripe.
Carol, when the good ones start dropping, be sure to remove the green husk material as soon as possible so it doesn't go all black and gooey and discolor the kernels(and impart a bitter taste).
Oh... and unlike their cousins the pecans & hickories, they are not ready to eat when they drop. Once you've removed the husks, you'll need to store them in a cool, dry place to cure for several weeks before they're good to eat.

This post was edited by lucky_p on Sun, Aug 17, 14 at 18:15

Thanks, friends; but I'm so totally clueless about the general maturity date of black walnuts that I don't know if I should be harvesting them very soon, or in mid-November. When are they ripe, in zone 6?

Carol