Kansa, the king of Mathura, was a cruel tyrant. After his sister Devaki married Vasudeva, Kansa himself was driving their chariot to see them off when a voice rang out from the sky: "O Kansa, the eighth child of the sister you are so lovingly sending away will be your death." The moment he heard this, Kansa threw Devaki and Vasudeva into prison.
One after another, Kansa killed six of Devaki's children. Yogamaya moved the seventh child into the womb of Rohini, Vasudeva's other wife, and from her Balarama was born. Then, at midnight on the eighth day of the dark fortnight of Shravan, on a dark and rainy night, Lord Vishnu appeared in the prison itself as Shri Krishna.
The moment he was born, miracles happened: Vasudeva's chains broke, the prison doors swung open and the guards fell asleep. Vasudeva placed baby Krishna in a basket and set out to cross the Yamuna. The river was in full flood, but as soon as it touched the infant Lord's feet the water fell away, and the serpent Shesha spread his hoods to shelter him from the rain.
That same night in Gokul, in the home of Nanda Baba, Yashoda had given birth to a baby girl. Vasudeva laid baby Krishna beside Yashoda, took the little girl and returned to the prison. When Kansa lifted the baby to kill her, she slipped from his hands, rose into the sky as a goddess and declared: "The one who will destroy you has already been born!" This day is celebrated today as Janmashtami.