Why we don't practice on Moon Days

• From Shri K. Pattabhi Jois at the Ashtanga Yoga Shala:
  "That day is very difficult day. Two stars one place (conjunction) is going. New moon also, full moon also. That day very dangerous day. You (take) practice (on that day), anyone can have a small pain starting. That pain is not going very quickly. Long time he is taking. Some broken possible. That is why that day don’t do." source
  
   
• From Richard Freeman at the Yoga Workshop:
  "Observing this restraint to practice can be helpful in not becoming too attached to practice and routine. It also provides time for the body to rest and recuperate." source 
  
  "It’s part of the traditional approach to take time off during the new and full moons. This is partly due to the Indian astrological belief that it is not auspicious to do certain things on moon days. Because we are part of this lineage, we have chosen to honor the moon days in this way.
  
  In addition, once you practice on a daily basis (six days a week is recommended), you’ll notice that being invited to take a day off is a luxury. The body can rest (after all the ashtanga practice is physically demanding) and on moon days you feel like you have a huge chunk of unspoken for “free time” when you’re used to daily practice.  source 
  
   
• From Tim Miller at the Ashtanga Yoga Center:
  "Like all things of a watery nature (human beings are about 70% water), we are affected by the phases of the moon. The phases of the moon are determined by the moon’s relative position to the sun. Full moons occur when they are in opposition and new moons when they are in conjunction. Both sun and moon exert a gravitational pull on the earth. Their relative positions create different energetic experiences that can be compared to the breath cycle. 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 Upanishads state that the main prana lives in the head. During the full moon we tend to be more headstrong.
  
  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.
  
  The Farmers Almanac recommends planting seeds at the new moon when the rooting force is strongest and transplanting at the full moon when the flowering force is strongest.
  
  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."  source

• From Eddie Stern at Ashtanga Yoga New York:

"I had one friendly comment to pass on about the ‘anandhyanana’ days: 

It is possible that the student who asked you about any prohibition of practicing yoga on the full or new moon days was doing so because of the observances of Pattabhi Jois. Much has been made of this observance, with all sorts of ideas about why he does this, and what significance it may have. However, the matter is quite simple. As you know, the Maharaja’s Pathashala (Sanskrit College) was closed each month for classes on the moon days, and the day before and after. Studies were continued by the students, but no new lessons taught. One reason for this was that on amavasya and purnima, certain rituals had to be performed by the teachers and students alike, who are all brahmins – for example, the pitr tarpana which needs to be performed on amavasya, and the ritual bathing the day after the moons – all these things take time to be performed. As well, though I have never been able to find the reference, Pattabhi Jois used to quote to us – and I also heard this from my old Bhagavad Gita teacher in Mysore – that if a teacher teaches new subjects on the moon days, his knowledge will decline, and on the day before or after, the knowledge of the student will decline! Perhaps you might know where this reference comes from? 

When I spoke to Pattabhi Jois’s astrologer while interviewing him for the Guruji book, he concurred with the idea that it has something to do with the idea of as above, so below: our mind is the moon, and waxes, wanes, and retains information in a similar cycle as the moon in the sky. 

Since Pattabhi Jois was a student at the Maharaja’s Pathashala, and then was the Professor of Yoga there from 1937 to 1973, this became a habit and observance for him. Since he held the view that yoga was a practice of Vedic origin, and that the knowledge of the Upanishads was to be accessed only through the doorway of asanas and pranayama, he ascribed the same observances to teaching them as he did to teaching Veda. He further used to say that on the full and new moon days, there was a particular conjunction of nakshatras that made it easier to get injured, and that the injury would take longer to heal. I have never been able to verify this through jyotish; perhaps this is something that he learned from his father, who was an accomplished jyotishi. 

Pattabhi Jois knew quite a bit too — the name Jois is a South Indian corruption of Jyotish, and astrology was in his family tradition. I say all this to make the simple point that Pattabhi Jois had certain habits from the time he was 14. Why he had these habits is interesting, and though we may not be brahmins, or even Indian, as his students it is good to understand why certain things were done by him, and accept that if he felt them important enough to follow, that they are applicable to us too. But we should not go making a big thing of it and creating all sorts of fantastical ideas! 

Below is a funny story to illustrate what happens when we (for example, Ashtanga Yoga students!) do not take the time to investigate simple things in a rational manner: 

A saintly scholar used to give a class on Bhagavad Gita each evening beneath a tree near a village. He had a pet cat, and this cat would sometimes run through the crowd, making a disturbance. As a result the sage began to tie the cat to the tree during the class. After some time the speaker shuffled off his mortal coil. One of his disciples continued to give the Bhagavad Gita class under the tree, and continued to tie the cat to the tree during the class. After some time the cat passed away, and the disciple bought another cat. After three generations a disciple wrote a paper on the sacred tradition of tying a cat to the tree while giving a class on Bhagavad Gita. 

So, all that being said, I think that the moon day/practice observance should be followed by the Ashtanga Yoga students out of respect for Pattabhi Jois and his methods. The purpose of following these things, and submitting ourselves to a lineage, is to create humility and thoughtfulness in the student. We will (most likely) not go to hell if we practice on these days, but surrendering oneself to a lineage has its own charm and effect on our character, so why should we not try it? I do not believe that all yoga students should refrain from practice on these days – they too should follow the observances of their teachers, and hopefully by aligning our minds with higher principles, we will all find happiness in our practices. On moon days or not!" source

• From David Miliotis at the Ashtanga Yoga Practice:
  "Why Vedic moon days differ from Western moon days:
  
  In Vedic astrology, the lunar cycle, is divided into 30 tithis; 2 of these tithis are called full (pūrṇimā) & new (amāvasyā) and have been loosely translated as full & new ‘moon days’. The tithi is a specific time period that begins and ends based on its lunar cycle - irrespective of the daily solar cycle. The Western method for determining full & new moon days is to simply considered on which day the moon happens to be exactly full or new. When Guruji would speak of full & new moon days, he was thinking of their respective tithis. For this reason, we too follow the Vedic Pañcāṅga calendar system - just as Guruji did." source