The story of Janmashtami is such that when the darkness on the earth started in Dwapar Yuga, gods requested Shri Narayan. He promised that he would take another avatar and this is where the story of the incarnation of Shri Krishna begins.
In the Rohini Nakshatra on the midnight of Bhado, Lord Shri Krishna was born in the conch, chakra, mas. Seeing this Leela of Narayan, Devaki and Vasudev were overwhelmed. After that, Shri Krishna gave instructions to Vasudev and came in child form.
After this amazing events took place in Kargar. Vasudev's handcuffs opened. All the doors were asleep. Vasudev picked up Krishna and walked towards Vrindavan.The Yamuna used to be on the way. That too Bhado's booming Yamuna. While Vasudev was crossing the Yamuna, Mother Yamuna rose to touch the feet of Krishna.
The Yamuna would go up as much as Vasudev lifted the basket. There came a time when the water level of Yamuna rose above Vasudev's head. After this, Shri Krishna hung his foot down and the Yamuna was bound to touch Charan.
Early in the middle of the night, Vasudev Maharaj reached Nanda's house and picked up his newborn girl and placed Krishna in his place and returned. After coming back, the whole scene of the prison is again like before. His handcuffs took place. The guards woke up.