Bhashya
ख्यं कर्म आरभ्यते महत्त्वप्राप्तये ; महत्त्वे च सति अर्थसिद्धंहि वित्तम् ।तदुच्यते—स यः कामयेत, स यो वित्तार्थी क-र्मण्यधिकृतः यः कामयेत ; किम् ? महन् महत्त्वम् प्राप्नु-याम् , महान्स्यामितीत्यर्थः ।तत्र मन्थकर्मणो विधित्सितस्यकालोऽभिधीयते— उदगयने आदित्यस्य ; तत्र सर्वत्र प्राप्तौआपूर्यमाणपक्षस्य शुक्लपक्षस्य ; तत्रापि सर्वत्र प्राप्तौ, पुण्याहेअनुकूले आत्मनः कर्मसिद्धिकर इत्यर्थः ; द्वादशाहम् , य-स्मिन्पुण्येऽनुकूले कर्म चिकीर्षति ततः प्राक् पुण्याहमेवा-रभ्य द्वादशाहम् , उपसद्व्रती, उपसत्सु व्रतम् , उपसदःप्रसिद्धा ज्योतिष्टोमे, तत्र च स्तनोपचयापचयद्वारेण प-योभक्षणं तद्व्रतम् ; अत्र च तत्कर्मानुपसंहारात् केव-लमितिकर्तव्यताशून्यं पयोभक्षणमात्रमुपादीयते ; ननु उप-सदो व्रतमिति यदा विग्रहः, तदा सर्वमितिकर्तव्यतारूपंग्राह्यं भवति, तत् कस्मात् न परिगृह्यत इत्युच्यते— स्मा-र्तत्वात्कर्मणः ; स्मार्तं हीदं मन्थकर्म ।ननु श्रुतिविहितंसत् कथं स्मार्तं भवितुमर्हति— स्मृत्यनुवादिनी हि श्रुति-रियम् ; श्रौतत्वे हि प्रकृतिविकारभावः ; ततश्च प्राकृतधर्म-ग्राहित्वं विकारकर्मणः ; न तु इह श्रौतत्वम् ; अत एव चआवसथ्याग्नौ एतत्कर्म विधीयते, सर्वा च आवृत् स्मार्तैवेति ।
In plain words
This rite — named on the page before this one — exists to make a man great, and only once he is great does his wealth actually mean anything. Scripture puts it plainly: whoever wants wealth, and is entitled to perform this rite, may desire — desire what? To attain greatness, to become great, that's the whole of it. As for when: during the sun's course to the north, in the waxing half of the month, on a day judged auspicious and favorable to his own success. Twelve days, counting back from that chosen day, he keeps the upasad-vow — the same vow known from the upasads of the great Jyotiṣṭoma sacrifice, where milking is carefully increased and then decreased, and drinking that milk is the whole of the vow. But here he takes only that one piece — plain milk-drinking, with none of the surrounding ritual detail — because the rest of that older rite was never meant to come along with it. Someone might object: read the compound the other way, as 'the vow belonging to the upasad,' and surely all its detail should transfer too — so why doesn't it? Because this rite rests on remembered tradition, not on direct scriptural injunction — that's the answer. But then comes the sharper question: how can something scripture itself describes fail to count as scriptural? Because this particular passage isn't laying down something new — it's simply repeating what tradition already held. Had it been a fresh Vedic injunction, the rite would stand in that strict archetype-and-copy relationship, obligated to inherit every feature of its model. That relationship simply doesn't hold here. Which is exactly why this rite belongs to the ordinary household fire, and its whole procedure is, start to finish, drawn from tradition rather than direct revelation.
Close to the Sanskrit
The rite called [mantha] is undertaken for the attainment of greatness; and when there is greatness, wealth indeed becomes fulfilled in its purpose. This is what is stated: 'whoever might desire' — that is, whoever, desirous of wealth and qualified for the rite, might desire; desire what? 'May I attain greatness' — that is, 'may I become great,' this is the meaning. There, the time for the mantha-rite one intends to perform is declared: in the northward course of the sun; there, in the general case, in the waxing fortnight, that is, the bright fortnight; there too, in the general case, on an auspicious day — favorable, that is, productive of the success of the rite for oneself — this is the meaning. 'The twelve-day period': on which auspicious, favorable day he wishes to perform the rite, counting back from that, beginning exactly with that auspicious day, twelve days. 'One who observes the upasad-vow' — the vow observed during the upasads, the upasads being well known in the Jyotiṣṭoma sacrifice; there, by means of increasing and decreasing the milking of the udder, the drinking of milk alone is that vow. But here, since that entire rite is not carried over, only the mere drinking of milk, empty of any further procedural content, is adopted. Objection: when the compound is analyzed as 'the vow of the upasad,' then the whole of the procedural content should be taken — why, then, is it not adopted? This is explained: because the rite is smārta. For this mantha-rite is indeed smārta. Objection: but how can something enjoined by scripture be smārta? This scripture merely restates what is already in smṛti; for if it were śrauta, there would be an archetype-and-modification relationship, and the derivative rite would then have to take on all the properties of the archetype rite — but that status does not apply here. For this very reason, this rite is enjoined in the āvasathya fire, and the whole liturgical procedure is indeed simply smārta.