Mudgalapurana 4.10.22

4.10.22

tatrā''dau mārgaśīrṣasthā samprāptā śuklagā nṛpa |
caturthī sā kṛtā tenopoṣaṇena yathāvidhi ||

Then first, O king, when the bright fourth day falling in Mārgaśīrṣa arrived, / it was performed by him through fasting according to rule.