The Moon is "void of course" when it has completed all its major aspects while in a specific sign. The Moon could be in any phase and also be in the last degrees of a sign. Void of course means without a course, or path, to aspect another planet. Generally the major aspects are thought to include conjunction, sextile, square, trine, and opposition.
Readers, please enjoy this guest blog post by Sandra Kynes, author of several books, including The Herb Gardener's Essential Guide, Plant Magic, and the new Witches' Encyclopedia of Magical Plants, among many others.
Although a few old books...