What Is a Void-of-Course Moon?
Most of the time the Moon is moving through a sign and still forming aspects to other planets along the way. A void-of-course window is the gap after its last aspect in a sign but before it crosses into the next one — traditionally read as a lull, not a lull in the sky itself.
How it's actually computed
This isn't a fixed lookup — it's found by scanning forward from right now in small time steps, tracking the Moon's real position against the Sun and five visible planets, until the Moon's longitude crosses into the next zodiac sign. Whichever step held the last exact major aspect (conjunction, sextile, square, trine, or opposition) before that crossing marks the end of the aspects; everything after it, up to the sign change, is the void window.
What tradition says to do with it
The old rule of thumb is to avoid starting new things — signing something, launching a project, sending a first message — while the Moon is void, on the idea that things begun then tend to fizzle or need redoing once it changes sign. It's not read as a bad omen for existing plans, just a poor window for firsts.
Check the current window
Void-of-course windows can run from minutes to over a day, and they're specific to right now, not to a whole day the way a moon phase is. Synastri's `/moon-phase` tool computes the real current window this way — current sign, next sign, and the exact time of ingress — instead of publishing a generic daily table. For entertainment.