Bhagavad-gita. 4.7-4.8
yadā yadā hi dharmasya glānir bhavati bhārata
abhyutthānam adharmasya tadātmānaṁ sṛjāmy aham
paritrāṇāya sādhūnāṁ vināśāya ca duṣkṛtām
dharma-saṁsthāpanārthāya sambhavāmi yuge yuge

Translation: Whenever and wherever, there is a decline in religious practice, O descendant of Bharata, and a predominant rise of irreligion – at that time I descend Myself. To deliver the pious and to annihilate the miscreants, as well as to re-establish the principles of religion, I Myself appear, millennium after millennium.
Srila Prabhupada writes that the word srijami in this verse is significant. He has translated it as “manifest” and explains that it cannot be translated to say that Krishna creates His form. Neither is He a soul who acquires a temporary material body as we do. Krishna’s forms or bodies are not created or acquired. They are forever existent. Whether He appears as four-armed Vishnu or as a smiling, effulgent little baby, He has the self-same perfect body throughout all time and circumstances. As He likes, He manifests Himself in His own particular way to deliver the suffering people in the material world.
Lord Krishna appeared in His original form, more than 5000 years ago, in the holy land of Mathura. At the time the world had been overrun by cruel kings and people were suffering under their unjust rule. To protect the innocents and His devotees and to punish the miscreants, Krishna decided to manifest as He is.
His parents were Vasudeva and Devaki and as they were returning home from their wedding, a mysterious voice in the sky announced that the eighth child born to them would defeat Kamsa, the cruel king of Mathura. This eighth child was none other than Lord Krishna. When King Kamsa heard the prophecy, he became determined to destroy Krishna. Kamsa locked up Krishna’s parents in a prison, and there Krishna’s mother gave birth to seven children, six of whom died by Kamsa’s cruel hand.
One elder brother of Krishna’s eluded the sinful king. Known as Balarama, He is a plenary expansion of the Godhead. He transferred Himself to another location and waited for Krishna to also go there. Finally, the eighth child of Devaki, Krishna, was born in the very dungeon of His family’s persecution.
Krishna entered this world through the heart of his father, Vasudeva, who had performed strong penances and austerities for thousands of years in his previous lifetimes. Vasudeva did this in order to achieve an intense meditation on the Lord by which He was able to attract Krishna to appear as his own son. Devaki, alongside her husband, had also engaged in long severe penances to attract the Lord. From Vasudeva’s heart Krishna was transferred to the womb of Devaki.
When Krishna first appeared to His parents, He showed Himself as the splendorous four-armed Lord Vishnu, upon whom they were accustomed to meditate. Mother Devaki prayed to Him:
I understand that this transcendental form is generally perceived in meditation by the great sages, but I am still afraid because as soon as Kamsa understands that You have appeared, he might harm You.… My only cause of fear from my brother Kamsa is due to Your appearance…. Therefore, I request You to conceal this four-armed form of Your Lordship, which holds the four symbols of Vishnu—namely the conch shell, the disc, the club and the lotus flower. My dear Lord, at the end of the annihilation of the cosmic manifestation, You put the whole universe within Your abdomen; still by Your unalloyed mercy You have appeared in my womb. I am surprised that You imitate the activities of ordinary human beings just to please Your devotee.

 

A Charming Infant Form

The Lord accepted Devaki’s request for Him to conceal His four-armed form. He became her baby—a sweet, absolutely charming manifestation of the supreme omnipotent Godhead. Like all of the Lord’s wonderful forms, this baby form is the source of unlimited ecstatic bliss and love of God, the impetus for His parents’ devotional service.
To hide Krishna from Kamsa, Vasudeva carried Him across the Yamuna River to the simple cowherd village of Gokula Vrindavana and left Him in the care of His surrogate parents, Yashoda Devi and Nanda Maharaja. Gokula is the complete spiritual world manifested on earth to serve Krishna. It is filled with myriad devotees, including the forests, animals, six verdant seasons, sparkling rivers, and unlimited varieties of sweet fruits and flowers.
Krishna thus grew up as a cowherd boy in a farm village. His childhood pastimes in this intimate location were a continuous celebration of sweet bliss for His devotees. The cowherd men and their motherly wives were so glad to be with the Supreme Personality of Godhead that not a day would go by without them enjoying many affectionate embraces, jokes, and games with Krishna in their mood of profound parental affection.
Prabhupada said that only with the eyes of love of Godhead can one see the real identity of Vrindavana as the place where Lord Krishna performs His pastimes with the cowherd boys and girls.
For the cowherd boys and girls who were Krishna’s peers, Krishna was the inspiration in all of their activities. In talking they talked of Him; in singing they sang to please Him. They decorated Him, danced with Him, and joked with Him in the same way that young people love to rejoice in one another’s company. All of these exchanges were of the highest and finest spiritual quality, nothing like the relationships of this world, which are generally self-centered and selfish. They wanted only to render service to Krishna. He and His devotees in Gokula Vrindavana experience the perfect loving sentiments exchanged among servants, parents, siblings, friends, and lovers.

Lord Krishna is always Transcendental and Supreme

Bhagavad-gita. 4.9
janma karma ca me divyam
evaṁ yo vetti tattvataḥ
tyaktvā dehaṁ punar janma
naiti mām eti so ’rjuna

Translation: One who knows the transcendental nature of My appearance and activities does not, upon leaving the body, take his birth again in this material world, but attains My eternal abode, O Arjuna.

The Lord’s appearance is not like that of an ordinary man, who is forced to accept a material body. He appears out of His own sweet will. At the time of Krishna’s appearance, the planetary systems were automatically adjusted so that everything became auspicious. Everywhere, there was an atmosphere of peace and prosperity. Auspicious stars were visible in the sky.

Lord Krishna is all-attractive, all-auspicious and all-blissful, and so are His pastimes, which are completely beyond the imperfections and contaminations of the material world and activities.

When one hears such transcendental pastimes of the Lord with full faith and devotion, from a devotee then one is directly associating with the Lord Krishna Himself.

 

Why does Krishna appear?

paritrāṇāya sādhūnāṁ
vināśāya ca duṣkṛtām
dharma-saṁsthāpanārthāya
sambhavāmi yuge yuge
(Bhagavad Gita as it is 4.8)

“To deliver the pious and to annihilate the miscreants, as well as to reestablish the principles of religion, I Myself appear, millennium after millennium.”
Krishna Himself answers that question in Bhagavad Gita.
We can see that the first reason is to protect the pious, His devotees. Lord Krishna is always very eager to reciprocate the love of His devotees and when His dear devotees were being harassed by the demon Kamsa, Lord Krishna decided to appear personally to protect them.
Krishna is the supreme well-wisher of all living beings. He is especially the protector of all His surrendered devotees who take exclusive shelter of His divine lotus feet. He relieves them from all sinful reactions, sufferings, and obstacles on the spiritual path. He protects whatever they have, and supplies whatever they need for their spiritual progress. The following verse describes His solemn vow to protect His devotees:
tasman mac-charanam gostham
man-natham mat-parigraham
gopaye svatma-yogena
so ’yam me vrata ahitah
“I must therefore protect the cowherd community by My transcendental potency, for I am their shelter, I am their master, and indeed they are My own family. After all, I have taken a vow to protect My devotees.” (SB 10.25.18)
The second reason is to destroy the demons and sinful kings, those who torture and disturb His dear devotees and the innocent people. Not only in Mathura and Vrindavan, but Krishna appears over and over again and kills His devotees’ enemies. While killing them, he actually gives them the greatest of blessings, for to be killed by the Lord is to be liberated from this world of duality and misery.
And the third reason is to re-establish the principles of religion. Under the influence of sinful beings, religion, morality and good virtues slowly diminish and general populace becomes bewildered and confused about the performance of proper duty and spiritual activities. For that reason, Krishna first speaks the knowledge of Bhagavad Gita to Sun God and then to Arjuna, who is His dear friend. To understand the supreme knowledge of Bhagavad Gita, we have to purify our hearts and minds by performing sadhana bhakti. Lord Krishna appears as Vyasadev, His literary incarnation and compiles many scriptures to help us navigate this hazardous world. He also teaches by His own example in His unlimited incarnations, on how to perform duty, follow dharma, be a righteous king, be a dutiful son, be the best friend, etc.

Observing Janmashtami

The appearance day of Lord Krishna, who the Vedic scriptures describe as the Supreme Lord, in this material world is celebrated as Janmashtami.

More than 5000 years ago, Krishna appeared at midnight on the 8th day of the waning moon (Ashtami tithi under Rohini Nakshatra). On this day devotees fast till mid-night and observe Janmashtami vrata and seek His blessings. One can celebrate the appearance of Lord Krishna in the following ways:

• Visit a Lord Krishna temple near one’s house to offer prayers and gifts, and/or organize a small gathering at your home.

• Offer flowers, Tulasi leaves or foodstuff to a picture of Krishna at home

• If one has Deities of Sri Sri Radha Krishna at home, one can perform abhishek with water or pancamrita, followed by offering foodstuffs and arati.

• Krishna and His name are nondifferent. Thus, chant the Hare Krishna Maha mantra: Hare Krishna Hare Krishna Krishna Krishna Hare Hare, Hare Rama Hare Rama Hare Rama Rama Rama Hare Hare. This mantra is recommended for this age of hypocrisy & anxiety (Kali-yuga). Krishna is easily attracted by those who chant His names with love and devotion.

 

Prayer

On Janmashtami one can pray to Krishna humbly requesting Him to appear in our heart and always attract our mind towards Him. May He free us from the material illusions of this world and bless us to purify ourselves so that we can get entrance into His spiritual home, Goloka Vrindavan, in the spiritual world and live with Him eternally.

Listen to the narration by Amala Bhakta das of the appearance of Lord Sri Krishna from the book “Krishna, The Supreme Personality of Godhead”.
https://www.youtube.com/watch?v=-J4MN7J5rik&list=PLYPnLZvIOH3zBfHMw-vaFCUPp_OHpgZp-&t=79s