Goddess Chaṇḍikā (Chandika), willing to see Rāmacandra (Ramachandra) victorious, decided that she will remain in pitṛsvarūpa (pitriswarupa; assuming the form of a fatherly deity) upto Amāvasyā (Amavasya). On those days, pitṛkārya (pitrikarya; rites dedicated to the forefathers’ spirits) can be performed as it is to be done on Amāvasyā.
Goddess Chaṇḍī, known as Amā, pervades all the tithi(s) beginning from the first tithi of the dark fortnight in the month of Āśvina (Aswina; more or less simultaneous with October).