Phaladeepika
विष्टिः स्थिरं वा करणं यदि स्यात् कृष्णं जयेत् पौरुषसूक्तमन्त्रैः । पष्ठ्यां गुहार्धनमत्र कार्यं यजेच्चतुर्थ्यां किल नागराजम् ॥ १६॥
If the astrological division of the day is either viṣṭi or a fixed karaṇa, one should worship Kṛṣṇa with the hymns of the Puruṣasūkta. On the sixth lunar day, one must perform the worship of Guha, and on the fourth day, one should indeed sacrifice to the King of the Nagas (Nāgarāja).
english translation
viSTiH sthiraM vA karaNaM yadi syAt kRSNaM jayet pauruSasUktamantraiH । paSThyAM guhArdhanamatra kAryaM yajeccaturthyAM kila nAgarAjam ॥ 16॥
hk transliteration by Sanscript