Traditionally ashtanga yoga is practiced every day of the week except Saturdays and moon days.
Why Saturday? I think way back when Guruji was teaching he actually taught every day of the week but his family convinced him that he needed to rest himself. Hence Saturdays of rest were born. But when I was in India, Saraswathi's students rested on Friday and took led Primary classes on Saturday so actually practicing on a Saturday is not strictly against the "rules."
Why moon days? From what I understand, mainly to honor the cycle of the earth. On full moon days the earth's energy is especially high and on new moon days especially low. So take rest on those days. Now moon days are non-negotiable.
Today's full moon was especially auspicious because of the lunar eclipse. While I didn't wake up in time to see the blood moon, I still rather enjoyed the sight of a full Mr. Moon early this morning. Hopefully this bodes for a good day.