That was the first day that Nityananda Prabhu joined Mahaprabhu’s party, and up to His last days Nityananda Prabhu preached Chaitanya Mahaprabhu’s glories. From that day Mahaprabhu and Nityananda Prabhu started mahasankirtan. Mahaprabhu appeared to inaugurate sankirtan-yajna, sacrifice in the form of sankirtan (congregational chanting), as described in the Srimad Bhagavatam:

kṛṣṇa-varṇaṁ tviṣākṛṣṇaṁ sāṅgopāṅgāstra-pārṣadam
yajñaiḥ saṅkīrtana-prāyair yajanti hi su-medhasaḥ

«In the Age of Kali, persons of fine theistic intellect perform the sacrifice of congregational chanting (sankirtan-yajna) to worship the form of the Lord who constantly sings nothing but the Names of Krishna and describes His glories with great pleasure. He is Krishna Himself appearing in a molten gold complexion born from supreme divine emotion. With His personal beauty, Divine Name, all-merciful nature, and benevolent associates, He destroys the darkness of ignorance in the mundane world.»

Mahaprabhu Himself gave the gist of the nature of Hari-nam-sankirtan:

cheto-darpaṇa-mārjanaṁ bhava-mahā-dāvāgni-nirvāpaṇaṁ
śreyaḥ-kairava-chandrikā-vitaraṇaṁ vidyā-vadhū-jīvanam
ānandāmbudhi-vardhanaṁ prati-padaṁ pūrṇāmṛtāsvādanaṁ
sarvātma-snapanaṁ paraṁ vijayate śrī-kṛṣṇa-saṅkīrtanam
(Sri Chaitanya-charitamrta: Antya-lila, 20.12)

[«Sri Krishna-sankirtan cleanses the mirror of consciousness, extinguishes the raging forest fire of material existence, shines moonlight on the evening lotus of good fortune, is the life of the potency of divine knowledge, expands the ocean of ecstasy, is the taste of full nectar at every moment, and soothes the entire self. May Sri Krishna-sankirtan be supremely victorious!»]

That param vijayate Sri Krsna-sankirtanam, that supremely victorious sankirtan of Mahaprabhu, started with Nityananda Prabhu’s arrival, and in such a glorious way continued day by day in Sri Nabadwip Dham.