Revākhaṇḍa ascribed to the Vāyupurāṇa (RKV) 62.7

62.7

vikhyātaṃ tu tadā loke pāpaghnaṃ pāṇḍunandana /
aṣṭamyāṃ ca caturdaśyām ubhau pakṣau ca bhaktitaḥ /
upoṣya śūlinaścāgre rātrau kurvīta jāgaram //

It was renowned then in the world as destroying sin, O son of Pāṇḍu.
On the eighth and fourteenth days in both fortnights, having fasted with devotion,
one should keep a vigil at night before the wielder of the trident,