This term does not mean that the moon is invalid and everyone knows it but you. Void of Course refers to the time the moon spends between astrological signs. The term “void” refers to empty, and “course” refers to the path the moon travels. Technically, when the moon is void of course it means it has left the last aspect of one sign, and has not yet achieved the first aspect of the next sign. When you’re attempting to use lunar energy and the moon is between signs in this way, clearly defined lunar energy isn’t accessible.
Think of it as someone going through a tunnel when you’re trying to talk to them on a cell phone: their signal cuts out and you can’t communicate until they’re back on the open road again. It’s the same with the moon
A void can be anywhere between a few minutes to nearly two-day long. The best thing to do is check an astrological calendar for lunar positions. A regular wall calendar or agenda usually tells you the moon phase, but nothing more. Pick up a copy of the Farmer’s Almanac and you’ll be able to plot out spells.
Can you perform a spell when the moon is void of course? Of course you can. You’d be better to draw on something other than lunar energy to power it, because otherwise your efforts are likely to fizzle.