Calculating the times…
What a void-of-course Moon is
The Moon spends a little over two days in each sign. While it's there, it forms exact angles with the Sun and the planets, which astrology calls aspects. After its last exact aspect in a sign, and until it moves into the next one, the Moon is void of course. That gap can last a few minutes or more than a day.
Astrologers usually treat it as a better time for finishing, reviewing, and resting than for starting something new. It's a tradition, not a rule, so take it as a guide.
How the times are calculated
We count the five major aspects (conjunction, sextile, square, trine, and opposition) the Moon makes with the Sun, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. A period starts at the Moon's last exact aspect before it changes signs and ends when it moves into the next sign. Some astrologers count only the planets you can see with the naked eye, so other calendars may show different times.