This was a letter written by Eddie Stern of Ashtanga Yoga New York and Broome Street Temple that I found on facebook-wonderful explanation about moon days.
Eddie Sterns letter to Barry Silver about moon 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 reason for Pattabhi Jois's observance of these days is quite simple. As you know, the Maharaja's Pathashala (Sankrit 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 Bhagavad Gita teacher in Mysore,Professor Narayanacharya, - 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': in the Vedic tradition our mind is the like the moon, and waxes, wanes, and retains information following the same cycle as the moon in the sky exerts a gravitational pull on the earth.
Since Pattabhi Jois was a student at the Maharaja's Pathashala, and was the Professor of Yoga at the college from 1937 to 1973, taking those days off from teaching 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 through the doorway of asanas and pranayama, he ascribed the same observances to teaching yoga 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 about astrology, 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 out of respect for Pattabhi Jois, his methods and teachings, it is good for his students to follow the moon day observance, if they can. The purpose of following these things, and submitting oneself to a lineage, is to create humility, thoughtfulness and a certain type of discipline in the student. We will (most likely) not go to Hell if we practice on these days; Pattabhi Jois's daughter, Saraswati (who was the first and only woman to practice yoga with him at the Sanskrit College) used to teach her students Monday thru Friday and take weekends off, and said that on moon days she simply did not teach new poses. Also, she noted that her students did not practice everyday of the week, but for those of us who do, an occasional rest day is good for the body.
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!
Director: Ashtanga Yoga New York & Broome Street Temple