By far the most important influence on whether or not a spell has its intended effect is the current phase of the moon. While the exact reason is difficult to explain, it is easy to demonstrate—just observe the change in behavior patterns that occurs during the full moon. For centuries, people carefully observed and recorded the phases of the moon—early civilizations used the moon's phases almost exclusively to mark the passage of time and build their society and religious practices around these events.

With regards to the phases of the moon, the four distinctions relevant to witchcraft spells are: new moon, waxing moon, full moon, and waning moon. Different spells can have vastly different effects depending on when they are cast. In general, spells are categorized based on how closely they involve the spiritual world. As the moon waxes, so does both magic's potency and its potential to go awry. Using the minimum potency required for your spells is recommended.

The full moon is the time when the spirit world and the “human” world are the closest (save for certain yearly phenomena which vary in importance depending on the region you're located in). As a result, most magic spells are at their strongest. Summoning a spirit from the dead, or performing natural healing spells, are stronger in power due to the requirements of spirit intercession in the human world. On the other hand, if your spell has no effect on the spiritual world (for instance, a spell designed to get you money or fame, both material goals) or is used to reduce its influence on the material world (exorcisms or ridding oneself of a curse), it is not designed for this time. Sorcerers and witches in training are also better off practicing their spells during the full moon due to its effects on magic. Still, this should only be undertaken with supervision by experienced mentors, as more magic equals more opportunities for the spell to go awry. You should never cast a spell designed to rid oneself of unwanted spells or supernatural influence during the full moon, as it can exacerbate the problem.

The waxing moon is a time when magic's power is increasing, but not as strong as during the full moon. This takes the improved magical potency required for significant spellcasting and discourages wicked or mischevious spirits that appear more often during the full moon to sabotage the spell's workings. As a result, it is best for spells that have material rewards and do not affect the spiritual world strongly. For example, if you have a spell that earns you money, it is best to cast it during that time.

The waning moon and the new moon are interchangeable for spellcasting. In general, they are used for spells that reduce the influence of the spiritual world on the material world. Dispelling hexes that have been cast on you, for instance, is a common spell that is cast during this time. Or it can be used for ridding oneself of an unwanted spirit that has become attracted to magic you have done during other phases of the moon.

In addition to the moon, other celestial bodies can have a powerful impact on the spell you are casting. Properly annotated spells will have their requirements listed, though those requirements can vary by region. Still, many of these spells are old enough and have been passed down through so many different practitioners that information may get lost. You may have to vary the conditions in order to get best results. When in doubt, experiment. Magic is all about your personal intuition.