"Moondays" are the days of the Full or New Moon.
In Ashtanga Vinyasa Yoga, it is not recommanded to practice challenging Asanas on these days.
Balancing for these days are special
On these days we will follow the a short sequence of Asana & Mudras and the
New-Moon and Full-Moon series with Meditation, which Sri Sribhashyam recommanded for these days.