The Law Code of Yājñavalkya 1.3.120

1.3.120

strīṇāṃ garbhadhāraṇayogyāvasthopalakṣitaḥ kāla ṛtuḥ | sa ca rajodarśanadivasād ārabhya ṣoḍaśāhorātraḥ | tasmin ṛtau yujmāsu samāsu rātriṣu, rātrigrahaṇād divasapratiṣedhaḥ, saṃviśed gacchet putrārtham | yugmāsv iti bahuvacanaṃ samuccayārtham | ataś caikasminn api ṛtau apratiṣiddhāsu yugmāsu sarvāsu rātriṣu gacchet | evaṃ gacchan brahmacāry eva bhavati | ato yatra brahmacaryaṃ śrāddhādau coditaṃ tatra gacchato 'pi na brahmacaryaskhalanadoṣo 'sti | kiṃca parvāṇy ādyāś catasras tu varjayet | parvāṇīti bahuvacanād ādyārthāvagamād aṣṭamīcaturdaśyor grahaṇam | yathāha manuḥ |

The fertile period (ṛtu) is the time characterized by a woman's fitness for conception. And that comprises sixteen days and nights beginning from the day of the appearance of menses. During that fertile period, on even nights—by the mention of "nights", daytime is forbidden—he should approach, that is, go [to her], for the sake of a son. The plural in "on even [nights]" indicates inclusion; and therefore, even in a single fertile period, he should go on all the even nights that are not forbidden. Going [to her] in this manner, he remains indeed a celibate student. Therefore, where celibacy is enjoined, such as in an ancestral rite (śrāddha), even if he approaches [her], there is no fault of lapse from celibacy. Moreover, he should avoid the parvan days and the first four [nights]. Because "parvan days" is plural, implying "and others", the eighth and fourteenth days are included, as Manu says: