OK, being serious now since you asked, occult rituals tend to take place during full moon cycles which is when lunar energy is at it's fullest. Lunar energy which is also where we derive the word lunacy from is often deceptive yet powerful, it shows a reflection of the true light of the sun but in phases. Depending on where other planets are in arrangement, there can be great power to effect change for either good or ill. This last full lunar cycle corresponded with the almost (astrological) merging of Saturn and Jupiter causing a rare Christmas star event and during the full moon was an important and powerful occultist time frame in which to do castings (spells). And that is what High Grand Climax is referring to.