SB 10.5.1-2

śrī-śuka uvāca
nandas tv ātmaja utpanne
 jātāhlādo mahā-manāḥ
āhūya viprān veda-jñān
 snātaḥ śucir alaṅkṛtaḥ
vācayitvā svastyayanaṁ
 jāta-karmātmajasya vai
kārayām āsa vidhivat
 pitṛ-devārcanaṁ tathā
Synonyms: 
śrī-śukaḥ uvāca — Śrī Śukadeva Gosvāmī said; nandaḥ — Mahārāja Nanda; tu — indeed; ātmaje — his son; utpanne — having been born; jāta — overwhelmed; āhlādaḥ — in great jubilation; mahā-manāḥ — who was great minded; āhūya — invited; viprān — the brāhmaṇas; veda-jñān — who were fully conversant in Vedic knowledge; snātaḥ — taking a full bath; śuciḥ — purifying himself; alaṅkṛtaḥ — being dressed very nicely with ornaments and fresh garments; vācayitvā — after causing to be recited; svasti-ayanam — Vedic mantras (by the brāhmaṇas); jāta-karma — the festival for the birth of the child; ātmajasya — of his own son; vai — indeed; kārayām āsa — caused to be performed; vidhi-vat — according to the Vedic regulations; pitṛ-deva-arcanam — the worship of the forefathers and the demigods; tathā — as well as.
Translation: 
Śukadeva Gosvāmī said: Nanda Mahārāja was naturally very magnanimous, and when Lord Śrī Kṛṣṇa appeared as his son, he was overwhelmed by jubilation. Therefore, after bathing and purifying himself and dressing himself properly, he invited brāhmaṇas who knew how to recite Vedic mantras. After having these qualified brāhmaṇas recite auspicious Vedic hymns, he arranged to have the Vedic birth ceremony celebrated for his newborn child according to the rules and regulations, and he also arranged for worship of the demigods and forefathers.
Purport: 

Śrīla Viśvanātha Cakravartī Ṭhākura has discussed the significance of the words nandas tu. The word tu, he says, is not used to fulfill the sentence, because without tu the sentence is complete. Therefore the word tu is used for a different purpose. Although Kṛṣṇa appeared as the son of Devakī, Devakī and Vasudeva did not enjoy the jāta-karma, the festival of the birth ceremony. Instead, this ceremony was enjoyed by Nanda Mahārāja, as stated here (nandas tv ātmaja utpanne jātāhlādo mahā-manāḥ). When Nanda Mahārāja met Vasudeva, Vasudeva could not disclose, “Your son Kṛṣṇa is actually my son. You are His father in a different way, spiritually.” Because of fear of Kaṁsa, Vasudeva could not observe the festival for Kṛṣṇa’s birth. Nanda Mahārāja, however, took full advantage of this opportunity.

The jāta-karma ceremony can take place when the umbilical cord, connecting the child and the placenta, is cut. However, since Kṛṣṇa was brought by Vasudeva to the house of Nanda Mahārāja, where was the chance for this to happen? In this regard, Viśvanātha Cakravartī Ṭhākura desires to prove with evidence from many śāstras that Kṛṣṇa actually took birth as the son of Yaśodā before the birth of Yoga-māyā, who is therefore described as the Lord’s younger sister. Even though there may be doubts about the cutting of the umbilical cord, and even though it is possible that this was not done, when the Supreme Personality of Godhead appears, such events are regarded as factual. Kṛṣṇa appeared as Varāhadeva from the nostril of Brahmā, and therefore Brahmā is described as the father of Varāhadeva. Also significant are the words kārayām āsa vidhivat. Being overwhelmed with jubilation over the birth of his son, Nanda Mahārāja did not see whether the cord was cut or not. Thus he performed the ceremony very gorgeously. According to the opinion of some authorities, Kṛṣṇa was actually born as the son of Yaśodā. In any case, without regard for material understandings, we can accept that Nanda Mahārāja’s celebration for the ceremony of Kṛṣṇa’s birth was proper. This ceremony is therefore well known everywhere as Nandotsava.