Janmashtami marks the birth of Krishna. According to the Bhagavatam, Krishna appeared at the stroke of midnight, in a prison cell in Mathura, to free the world from the tyranny of the demon-king Kamsa.
That is why, even today, devotees keep a joyful vigil and celebrate His birth at the exact moment of midnight — singing, ringing bells, and rocking a small cradle holding the image of baby Krishna. The darkness of the prison giving way to His radiant arrival is a symbol of light conquering darkness within every heart.