Manava Dharmashastra 1.66

1.66

yo manuṣyāṇāṃ māsaḥ sa pitṝṇāmaharniśam| kataradahaḥ katamā ca rātririti pravibhāgaḥ| idamahariyaṃ rātrirityeṣa vibhāgaḥ | pakṣayoḥ pañcadaśarātrisammitayorarddhamāsākhyayorvyavasthitaḥ| pakṣāśrita ityarthaḥ | ekaḥ pakṣo'haraparo rātristayośca bhinnasvabhāvatvānniyatakramatvācca viśeṣamāha | ahaḥ kṛṣṇaḥ pakṣaḥ | śuklaḥ pakṣaḥ śarvarī rātriḥ | 'karmaceṣṭābhya' iti yuktaḥ pāṭhaḥ | yathā 'svaprāyeti' | tādarthyameva viṣayabhāvena vivakṣitam | vṛttānurodhādata: saptamī |

One month (of men) forms the ‘day and night’ of the ‘Pitṛs’; and their division is by fortnights: the darker fortnight, conducive to activity, is ‘Day,’ and the lighter fortnight, conducive to repose, is ‘Night.’—(66)

Medhātithi’s commentary (manubhāṣya):
That which is the ‘month’ of men is a ‘day and night’ of the ‘Pitṛs.’ As regards ‘division’ as to which part is ‘day’ and which ‘night,’—this division, that ‘this is day and that is night,’ is determined ‘by fortnights,’ i.e., the fifteen days, which are known by the name of ‘half-month.’ That is, the said division is based upon fortnights: one fortnight is ‘day,’ and another fortnight is ‘night.’ In view of the fact that ‘day’ and ‘night’ differ in their character and occur in a fixed order of sequence, the author adds the following distinction:—The darker fortnight is ‘day,’ and the brighter fortnight, ‘night.’

The right reading (in view of the meaning intended) would be ‘Karmaceṣṭābhyaḥ’ (the Dative form), just as we have ‘svapnāya’; for the meaning meant to be conveyed is that the day is for the purpose of ‘activity.’ Hence the Locative ending in the text can be explained only as used on account of the exigencies of metre.—(66)