Why is ANUSTHAN done on Amavasya and Paurnima days?
On these two days the planetary position of the SUN and MOON is such that SUN’s energy is most on mavasya day and Moon is least and vice-a-versa on Paurnima day.SUN activates soul and MOON activates mind. It is easier to rinse our subtle body and activate our Causal body. On all other days we gather or accumulate deeds which are not conducive to natural-health.Hence those can be reduced or removed from the body by doing ANUSTHAN.