It has always been the tradition in Ashtanga Yoga to rest from asana practice on new and full moon days (tithis). When asked why we shouldn’t practice on these days, Guruji was fond of saying, “Two ‘planets’ [grahas] one place, very dangerous.” What is meant by this is that on these days, the sun and the moon are in a line relative to the position of the earth. Consequently, their gravitational forces are all combined, and thus the effect of the ‘plantets’ more pronounced. One definitive effect of this is that the ocean’s tides are higher and lower on these days. When āsana practice is done daily, rest days are important for regeneration; and the extra biweekly ‘moon day’ comes as a welcomed respite.
The full moon energy corresponds to the end of inhalation when the force of prana is greatest. This is an expansive, upward moving force that makes us feel energetic and emotional, but not well grounded.
The new moon energy corresponds to the end of exhalation when the force of apana is greatest. Apana is a contracting, downward moving force that makes us feel calm and grounded, but dense and disinclined towards physical exertion.
Practicing Ashtanga Yoga over time makes us more attuned to natural cycles. Observing moon days is one way to recognize and honor the rhythms of nature so we can live in greater harmony with it.