This was a letter written by Eddie Stern of Ashtanga Yoga New York and Broome Street Temple that I found on Facebook. It's a 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 w...