An Introduction to Magical Timing

I. Basic Lunar Timing
The most basic, most widely practiced, and most generally useful form of magical timing is timing by the Moon. In the old magical philosophy of the Renaissance, the Moon was said to gather up and mediate all the influences of the other planets and stars and whirling heavens and send them down to Earth; in practice, things certainly seem to work that way.
The first dimension of lunar timing follows the waxing and waning phases. If you're new to this, "waxing" means growing and "waning" means shrinking; the Moon is waxing from the new moon to the full moon, when night after night it turns from a thin crescent to a full circle, and waning from the full moon to the new moon, when night after night it turns from a full circle into a thin crescent. The rule here is simple. If your working is intended to make something grow and flourish, you do it during the waxing moon, and if your working is intended to make something fade away and vanish, you do it during the waning moon. All else being equal, the best time to do a working to make something grow and flourish is during the first three days after the new moon, and the best time to do a working to make something fade away and vanish is during the first three days after the full moon. The last three days before the new moon, the period called the Eld of the Moon, are suitable only for curses and other noxious spells; the same is true of the day of an eclipse. Since all my readers are smart enough to avoid doing something that stupid, we'll leave the matter at that.
II. Advanced Lunar Timing
If you want to kick it up the next step, pay attention to where the Moon is in the signs of the zodiac, and choose a day when the Moon is in a sign that corresponds to the element or planet best suited to your working. Let's say you're doing some protective magic -- making and consecrating an amulet of protection, for example, using herbs such as St. John's wort, which correspond to the Sun. Protective magic is a waxing-moon thing -- you want your strength and health and luck to grow and flourish -- so you do it during the waxing moon, and on a day when the Moon is in Aries (where the Sun is exalted) or in Leo (when the Sun is in his rulership).
What if the Moon is waning when it's in Aries and Leo? You get to wait a few months until one of those signs is in the waxing half of the lunar cycle. That's the thing about magical timing: the more powerful it is, the more picky you have to be in choosing a time to do it. This means you have to wait for the right time.
III. The Planetary Hours
A different approach uses the planetary hours and days. Every day of the week is assigned to one of the seven traditional planets. (A few thousand years from now, when we as a species have finished integrating the influences of Uranus and Neptune into our collective consciousness, we'll probably have nine-day weeks and a ninefold pattern of planetary hours, but until that happens, we'll still be using a seven-day week and seven planet in magic.) Each day is divided into twelve hours, and each night likewise; each hour is assigned to one of the planets.
This is where it gets complex, because the hours are equal to an ordinary 60-minute hour on two and only two days of the year, the days of the spring and autumn equinoxes. The magical day extends from sunrise to sunset, and however long that interval is, it's divided into twelve equal planetary hours. The magical night, likewise, extends from sunset to sunrise, and however long that interval is, it's divided into twelve equal hours. Today at the latitude of Rhode Island, for example, a daytime planetary hour is 75 minutes long and a nighttime planetary hour is 45 minutes long. You used to be able to get books with tables of planetary hours -- I have one, Llewellyn's Improved Perpetual Planetary Hour Book -- though most people these days use computer programs and apps instead.
The rule here is very simple. Figure out what planet governs your working, and do it in the day and hour of that planet. The results will be good. If you pay attention to the waxing and waning of the Moon as well, this will improve your results.
IV. Astrological Timing
This approach is much more advanced, much more powerful, and much more difficult, and you have to have a good working knowledge of traditional astrology to use it. Modern astrology definitely will not cut it! Your common or garden variety modern astrologer has forgotten that some astrological factors are unlucky and unfortunate, and that your intentions don't actually amount to much in the vast wheeling fires and circling suns of the universe.
To choose a good time for a working using astrological timing, you need to start by determining what planet rules that working. Then you need to find a time when that planet is well dignified -- that is, in its rulership or exaltation if possible, and if not, at least in its triplicity, face, or term; when the planet receives no negative aspects from any planet, and is not applying to a conjunction with the Sun or the malefics; when, if possible, the planet receives favorable aspects from at least one planet, or a conjunction with the Moon or one of the benefics; and when it is conjunct the ascendant or applying to the midheaven.
If you don't know what these terms mean, you don't know enough astrology yet to use this approach to timing, so don't worry about it. If you do know what all these terms mean, and you know how to calculate when the relevant factors apply, you're good to go. This is far and away the most powerful approach to magical timing -- potent enough, in fact, that all you have to do is make the talisman and then bless it during the right astrological time, and it's fully consecrated by the energies streaming down from heaven. That's how the Picatrix teaches you to make talismans, for example.
V. Other Modes of Timing
Are there other approaches to magical timing? Of course. The cosmos of the practicing mage is full of complicated cycles of power, rising and falling according to intricate rhythms, and there are various ways of seeing to it that you do your working at a time when those cycles will help you rather than hinder you. If you're working in a specific tradition of magical practice, follow its directions for timing -- they're not there for decorative purposes! Otherwise, use one of the approaches given here and you'll be fine.