Parva days are mentioned in Manu Smriti on multiple occasions. Here is one such instance:
4.150. Let him always offer on the Parva-days oblations to Savitri and such as avert evil omens, and on the Ashtakas and Anvashtakas let him
constantly worship the manes.
A different translation of the above verse and commentary on it can be found from here:
सावित्रान् शान्तिहोमांश्च कुर्यात् पर्वसु नित्यशः ।
पितॄंश्चैवाष्टकास्वर्चेन्नित्यमन्वष्टकासु च ॥ १५० ॥
sāvitrān śāntihomāṃśca kuryāt parvasu nityaśaḥ |
pitṝṃścaivāṣṭakāsvarcennityamanvaṣṭakāsu ca || 150 ||
On special days, he shall always offer the oblations to Savitṛ, and
also the Pacificatory Oblations; on the “Aṣtakas” and on the
“Anvaṣṭakas” he shall always worship the Pitṛs.’—(150)
Medhatithi's commentary on this verse explains that Amavasya and Purnima are considered as the Parva days.
The exact forms of the afore-mentioned oblations are now described.
‘Oblations to Savitṛ.’—Those that are offered to Savitṛ as the deity.
‘Special days;’—i.e., on the Moonless and the Full Moon Days—the said oblations are to be offered.
‘Pacificatory oblations—the oblations that are offered for the purpose
of averting evil.