Source: AstroProfile Void Of Course Moon Tables
Void of course is a term applied to the time the moon is between signs. For example, it is the time in hours or minutes of the moon is leaving the sign of Gemini, but not yet moving into the sign of Cancer. It can be a few minutes or 2 days.
Traditionally, the void course of the moon is considered a time of chaos, disruption, when nothing works right, and times when we make more mistakes and bad decisions. The old saying exists with respect to the void course of the moon:
Whatever is done in a void of course moon has to be done again, repeatedly; and that which you want to repeat will never have a chance to manifest.
Of course, this all assumes one is powerlessness in the midst of chaos (tohu). In the case where the power of tohu has been harnessed and mastered, the void course of the moon is a goldmine of a time. A veritable goldmine. A treasure.