Definition: In Ashtanga yoga, daily practice is encouraged, with the exception of moon days. On days of the full and new moons each month, Ashtangis take a day off to rest. Pattabhi Jois believed moon days were dangerous and to practice on these days increased the risk of injury.
Source: Ashtanga Yoga Shala NYC.
Examples:
Since it is a moon day, I will not be attending Ashtanga yoga today.
