In Ashtanga Yoga, classes are traditionally held six days a week: Sunday through Friday. Students take practice all six days. Full moon and new moon days are the exception.
Students should take rest on full and new moon days. During the full and new moon, the gravitational pull of the sun and the moon are combined. The tides of earth's oceans are highest and lowest on moon days.
It is believed that practicing on moon days can lead to more serious injuries. Additionally, injuries that occur from practicing on moon days may take longer to heal. In any case, Ashtanga Yoga is a physically demanding practice and you will look forward to resting on moon days.
Full moons and new moons occur about twice a month.