Agniveshyagrihyasutra 2.7.9.30

2.7.9.30

ata ūrdhvaṃ sopavāsaḥ kāryaḥ dve triṣu ca tisra ityevamatīteṣu prāyaścittaṃ pārvaṇavaddhomaḥ pitṛyajñe 'tīte pakṣadvaye copavāsaḥ kārya iti siddham \

Beyond that, [it] is to be performed accompanied by fasting: two [oblations] when two [parvans have passed], and three when three. In this way, when they have passed, the expiation is an offering like that of the parvan. When the Ancestor-sacrifice has passed, fasting is also to be performed in both fortnights; this is established.