Śatapatha Brāhmaṇa ("of a hundred paths") 2.1.1.13

2.1.1.13

tadāhuḥ | ṣaḍevartavaḥ saṃvatsarasyeti nyūnamu tarhi mithunam prajananaṃ kriyate nyūnādvā imāḥ prajāḥ prajāyante tacvaḥśreyasamuttarāvattasmātpañca bhavanti yadyu ṣaḍevartavaḥ saṃvatsarasyetyagnirevaiteṣāṃ ṣaṣṭhastatho evaitadanyūnam bhavati

Now, as to this, they say, 'Six seasons there are in the year.' And in that case the very deficiency (nyûna) itself is rendered a productive union, since it is from the lower part (nyûna, i.e. of the body) that offspring is here brought forth. Thus also a progressive improvement (is assured to the sacrificer): for this reason there are five equipments. And when (it is nevertheless insisted on that) there are six seasons in the year, then Agni is the sixth of them, and thus there is no deficiency.