agnihotraṁ ca darśaś ca
paurṇamāsaś ca pūrva-vat
cāturmāsyāni ca muner
āmnātāni ca naigamaiḥ
The vānaprastha should perform the agnihotra, darśa and paurṇamāsa sacrifices, as he did while in the gṛhastha-āśrama. He should also perform the vows and sacrifices of cāturmāsya, since all of these rituals are enjoined for the vānaprastha-āśrama by expert knowers of the Vedas.
Purport:
Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura has given a detailed explanation of the four rituals mentioned here, namely agnihotra, darśa, paurṇamāsa and cāturmāsya. The conclusion is that everyone should simply chant Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare and avoid the difficult entanglement of Vedic ritualistic ceremonies. If one neither chants Hare Kṛṣṇa nor performs such rituals, one certainly becomes a pāṣaṇḍī, an atheistic fool.