The dark of the moon refers to the end of the lunar cycle when the moon is absent from the sky. That is, the period of time between the fading of the old, waning crescent moon and the appearance of the young, waxing crescent moon. This can last for 1.5 to 3.5 days.
Some traditions hold the dark moon period to be the three days prior to the appearance of the new moon.
Astronomically, however, the new moon occurs right in the middle of this period when the Sun and the Moon are in conjunction.
The Dark Moon is celebrated as the Hecate’s Deipnon in some traditions. Some Goddess-Centered traditions honor the Triple Goddess in Her Crone aspect at this time.
The dark moon is often a time designated by magic-users to clean their altars and perform rituals to cleanse their homes, ritual tools and auras. Regular maintenance banishing of negative energy or unwanted influence may be performed at this time.