There was once a cruel king called Kamsa. His sister Devaki married a nobleman called Vasudeva. A mysterious voice told Kamsa that he would be killed by a boy, the eighth child of Devaki and Vasudeva. Kamsa had Devaki and Vasudeva locked in prison for many years. During that time the couple had seven children and Kamsa had each of them put to death.
One dark and windy night, at midnight, an eighth child - a son - was born. A strange voice told Vasudeva to take the baby boy (whom they named Krishna) to the house of Nanda, chief of the cowherds, and his wife Yashoda and to bring back the baby girl he would find there. Miraculously, the guards did not wake up and the prison doors opened by themselves. Eventually, Vasudeva reached the village of Gokul where Nanda and Yashoda lived and he quietly exchanged the babies while everyone was asleep.
The next day Kamsa heard that a son had been born to Devaki and Vasudeva. Imagine the amazement when he discovered that the baby was a girl!. Baby Krishna was brought up by Yashoda and Nanda. Despite many efforts by Kamsa to kill him, Krishna grew up to show people how they should love God and live a dutiful life.
From Religion through Festivals: Hinduism