The Moon is void-of-course is when it has left its last major aspect in a sign, and has yet to enter a new sign. On a planetary calendar, it's noted as v/c.
It's a time when Moon energies are more free floating, and it's harder to find traction. Actions taken when the Moon is Void of Course may not catch either, since decisions are made from an ungrounded place.
See also Moon Void of Course.