Moon days in Ashtanga yoga

Traditionally in Ashtanga Yoga, we take both the full and the new moon off.  We call these days 'moon days'.

Why do we take moon days off?  Now days we do it to respect the traditions of Ashtanga yoga, and to respect our masters, specifically Sri K Pattahbi Jois (Guruji).  Guruji used to teach at the Sanskrit College at the Mysore Palace in Mysore, and in that college, Sanskrit classes were cancelled on the days of the full and new moon.  When Guruji later started his own shala, he kept these days off, originally taking only two days off work each month.

Why was the Sanskrit college at the Mysore Palace closed on moon days?  To answer this, we have to look at the role of the moon in primitive society.

Imagine living in darkness before electricity.  When nightfall comes, there's not much to do.  It's DARK!  Now have you ever been out to the country, or in a small town?  By the light of the full moon you can walk about and do stuff!  By the light of the full moon you can read and write. 
In many primitive societies, the full moon was a time to party, or a time to sow the crops, depending on the society.  More people and more activity means more mishaps and accidents and more dogs barking.  More light streaming through your bedroom window probably means it'll be harder to sleep too.  So if even in western society, we have this cultural artifact that people often think that they go a bit crazy around the full moon, because not too long ago, we were all partying!

India was at that time a very primitive society, superstitious and polytheistic.  Electricity only came to Bangalore in 1906, and slowly spread across major population centres in India over the next 30-40 years.  Now I don't purport to know why the Sanskrit classes were cancelled on moon days.  Perhaps everyone was out sowing the crops, or preying in temples.  Perhaps they were out on the town getting wasted! (In my research I have however learned a lot about Indian history - yay!)

Do superstitions die overnight in a culture with limited education?  Absolutely not!  You only have to look around you in educated societies for how omnipresent these superstitions and cultural artifacts are.  Think of how many people still see the number 13 as unlucky.  Do you know people who won't walk under a ladder?  Or even more common, how many people still say "bless you" after somebody sneezes?  I do, it's good manners.

People have tried to think up all sorts of things about the moon.  One common myth is that because our bodies are made of 70% water, they're affected by the moon like tides in the oceans.  This is a bit silly.  Any large body of liquid is affected by gravity, not specifically water, and we certainly don't have large expanses in our tiny human bodies.  Finally, we can turn to science to help us answer the question if the moon has an effect on us.  Here is a link to a site summarising a comprehensive list of studies.  These studies show that there is no significant link between the moon phase and human behaviour.

Guruji was a pragmatic guy, and would occasionally move the moon day if he had something to do and needed a day off.  Alan Little, in his diary from studying with Jois in 2001 joked when Mr Jois moved the moon-day "the ability to move the moon being one of those siddhis possessed only by a handful of advanced yoga masters".

Your yoga teacher may tell you that the moon day is special "because our bodies are made of water", or "because everyone gets a bit excited during a full moon and can hurt themselves".  They may say "Moon days are a cute part of the history of Ashtanga" and not observe them at all.  My yoga teacher says "It's a moon day, lets chill out a bit" then cuts a couple of Navasanas, drops Marichyasana D and Setu Bandhasana, and takes it easy in Urdhva Dhanurasana.

Each of these choices is cool with me.  Whatever they choose, it becomes part of their yoga practice.

So if you want to take the moon day off, go for it.  If you want to practice on a moon day, go for it.  I personally take the the moon day off.  Partly because I wish to show respect to the great master Pattabhi Jois and the traditions of Ashtanga yoga, and partly because I like an extra couple of days off each month.