01What Janmashtami is
Janmashtami, Krishna Janmashtami or Gokulashtami, is the eighth day of the dark half of Bhadrapada (Shravana in the south's reckoning), kept as the birth of Krishna, the eighth avatar of Vishnu. The tradition places the birth at midnight under Rohini nakshatra, so the worship centres on Nishita, the muhurta around midnight. The next day, in Maharashtra and beyond, is Dahi Handi.
02The story told of it
The Bhagavata Purana tells that Krishna was born at midnight in a prison cell in Mathura, where King Kamsa held his sister Devaki and her husband Vasudeva. That night the guards fell asleep and the doors opened, and Vasudeva carried the newborn across the flooded Yamuna in a basket to Gokul, where Nanda and Yashoda brought him up. The festival keeps the hour of the birth.
03How it is kept
Devotees fast through the day, often without grain, and the temples and homes are decorated with flowers and swings. At Nishita the image of the infant Krishna is bathed in milk, curd, ghee, honey and water, dressed and laid in a cradle, and the birth is celebrated with conch shells, bells and bhajans. Panjiri, makhan and misri are offered, and the fast is broken after the midnight puja or the next morning.
04How the day is chosen
Janmashtami is kept on the day Ashtami is current at Nishita, the muhurta around midnight, since the tradition places Krishna's birth there; when the next day has Ashtami at sunrise and Rohini nakshatra with it, and the first midnight has no Rohini, that next day is chosen. The fast is broken after sunrise once Ashtami and Rohini are both over.
05Janmashtami in other years