Janmashtami is the appearance day celebration of Lord Shri Krishna, the Supreme Personality of Godhead according to the Vedas, the ultimate authority on all matters of religion and life. The Lord came to this earth around five thousand years ago, towards the end of the Dvapara Yuga, in Mathura, India. Born as the eighth son of Mother Devaki, the Lord specially came to deliver His devotees. Technically, the Lord doesn’t take birth since He is eternal. Therefore, the birthday of Krishna is referred to as His appearance day.
"My dear Lord, it is not a very wonderful thing that You appear within the womb of Devaki because the creation was also made in that way. You were lying in the Causal Ocean as Maha-Vishnu, and by Your breathing process, innumerable universes came into existence.” (Vasudeva speaking to Krishna, Krishna, The Supreme Personality of Godhead, Volume 1, Ch 1.3)
The Lord’s appearance day is celebrated by the devotees since they love to always think of the Lord and remember His pastimes. Similar to the way we celebrate birthdays, anniversaries, and other holidays, devotees celebrate the anniversary dates of occasions related to God. In the Bhagavad-gita, Lord Krishna declares that He comes to earth from time to time to reinstitute the principles of dharma and to give protection to His dependents.
During the time of Krishna’s advent, there was a king named Kamsa who had amassed a great empire. Everyone lived in fear of Him due to His strength. A person of a demoniac nature, Kamsa had imprisoned his sister Devaki and her husband Vasudeva due to a prophecy that stated that Kamsa would killed by the eighth son of Devaki. As Devaki gave birth to her children, one by one Kamsa would mercilessly kill them by throwing them against a stone wall. In this way, he anxiously awaited the birth of Devaki’s eighth son. Krishna, seeing this predicament, decided to appear in the womb of Devaki to fulfill the prophecy and to free Devaki and Vasudeva. After being offered prayers from the demigods, Devaki gave birth to Krishna, who then appeared in His four-handed form of Narayana, or Vishnu.
“Thus eulogized by the gods, Devaki bore in her womb the lotus-eyed Lord Krishna, the protector of the world. On the day of His birth, the quarters of the horizon were filled with joy, as if moonlight was diffused over the whole earth. The pious experienced new delight, the strong winds were hushed, and the rivers glided tranquilly when Krishna was about to be born. The seas with their own melodious murmurings provided the music, while the Kinnaras and Gandharvas danced and sang and the demigods showered down flowers upon the earth. At midnight, when the supporter of all was about to be born, the clouds emitted low pleasing sounds and poured down a rain of flowers.
As soon as Vasudeva beheld the child of the complexion of lotus leaves, having four arms, and the shrivatsa mark on His chest, he addressed Him with love and reverence saying, ‘I understand that You have appeared to kill the uncivilized Kamsa and his followers. But knowing that You were to appear to kill him and his followers, he has already killed so many of Your predecessors, elder brothers. Now he is simply awaiting the news of Your birth. As soon as he hears about it, he will immediately appear with all kinds of weapons to kill You.’ Devaki also exclaimed, ‘My only cause of fear from my brother Kamsa is due to Your appearance. My Lord Madhusudana (Krishna), Kamsa may know that You are already born. Therefore I request You to conceal this four-armed form of Your Lordship which holds the four symbols of Vishnu. 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.’
To these pleas Krishna answered and said, ‘Dear mother, in former times I was prayed to by you and adored in the hope of progeny: your prayers have been granted, for I am born as your son. I know you are very concerned about Me and afraid of Kamsa. Therefore I order you to take Me immediately to Gokula and replace Me with the daughter who has just been born to Yashoda.’ So saying, Vasudeva, taking the baby, went out that same night; for the guards were all charmed by Yogamaya, as were the warders at the gates of Mathura, and thus they didn’t obstruct Vasudeva’s path. To protect the infant from the heavy rain that fell from the clouds of night, Ananta Sesha Naga, the supporter of the universe, the serpent with unlimited hoods, followed Vasudeva and spread his hoods above their heads; and when Vasudeva, with the baby Krishna in his arms, crossed the Yamuna river, deep as it was, and dangerous with numerous whirlpools, the waters became stilled and cleared a path for Vasudeva. On the bank he saw Nanda and the rest who had come there to bring taxes due to Kamsa. At the same time Yashoda was also under the influence of Yogamaya, whom she had brought forth as her daughter, and whom the prudent Vasudeva took up, placing Krishna in her place by the side of Yashoda. He then quickly returned home. When Yashoda awaoke, she found that she had delivered a boy, as black as the dark leaves of the lotus, and she greatly rejoiced.” (Story of Krishna’s birth found in the Vishnu and Bhagavata Puranas)