Matsya Purana — The Sun-Vow
तदाराध्य पुमान्विप्र प्राप्नोति कुशलं सदा तस्मादादित्यवारेण सदा नक्ताशनो भवेत् //
tadārādhya pumānvipra prāpnoti kuśalaṃ sadā tasmādādityavāreṇa sadā naktāśano bhavet //
O brāhmaṇa, by worshipping Him (Āditya, the Sun) a man always attains well-being and prosperity; therefore, on Sundays one should regularly observe the discipline of naktāśana, eating only at night.
This verse is not about Pralaya; it teaches Vrata-dharma—Sun-worship (Āditya) and a Sunday discipline of naktāśana to gain continual well-being.
It recommends a practical ethical-ritual discipline: regular Sun-worship and restrained eating on Sundays, a householder-friendly vow believed to support health, auspiciousness, and steady prosperity—qualities valued for both rulers and householders.
Ritually, it prescribes an Āditya-related vrata: worship of the Sun and naktāśana (eating only at night) on Sundays; it does not specify Vāstu or temple-construction rules in this line.