The Moon makes a loop around the Earth in about twenty-eight days, moving through each of the signs in two-and-a-half days (or so). As she passes through the thirty degrees of each sign, she "visits" with the planets in numerical order by forming angles, or aspects, with them. Because she moves one degree in just two to two-and-a-half hours, her influence on each planet lasts only a few hours, then she moves along. As she approaches the late degrees of the sign she's passing through, she eventually reaches the planet that's in the highest degree of any sign, and forms what will be her final aspect before leaving the sign. From this point until she actually enters the new sign, she is referred to as void-of-course, or void.
Think of it this way: The Moon is the emotional "tone" of the day, carrying feelings with her particular to the sign she's "wearing" at the moment. After she has contacted each of the planets, she symbolically "rests" before changing her costume, so her instinct is temporarily on hold. It's during this time that many people feel "fuzzy" or "vague"—scattered, even. Plans or decisions we make now will usually not pan out. Without the instinctual "knowing" the Moon provides as she touches each planet, we tend to be unrealistic or exercise poor judgment. The traditional definition of the void Moon is that "nothing will come of this," and it seems to be true. Actions initiated under a void Moon are often wasted, irrelevant, or incorrect—usually because information is hidden or missing, or has been overlooked.
Although it's not a good time to initiate plans, routine tasks seem to go along just fine. However, this period is really ideal for what the Moon does best: reflection. It's at this time that we can assimilate what the world has tossed at us over the past few days.
On the lighter side, remember that there are other good uses for the void Moon. This is the time period when the universe seems to be most open to loopholes. It's a great time to make plans you don't want to fulfill or schedule things you don't want to do.
From Llewellyn's 2001 Moon Sign Book. For current-year Moon Sign Book and more Llewellyn almanacs and calendars, click here.