On new moon days, bodily fluids decrease and joints are therefore less lubricated. In addition, since the body is drier, there is a greater risk of injury during practice.
On full moon days, bodily fluids are overstimulated. This results in more mobility when doing a pose, which makes the body more prone to injury due to the excessive oscillation of the joints. Note that a full moon injury is more serious and takes longer to heal. Consequently, practice during days of full moon or new moon is not recommended.