How the date is fixed
Krishna Janmashtami falls on the eighth day of the dark half of the lunar month Shravana (Krishna Ashtami). The date is the day on which that tithi is running at the middle of the night (halfway from sunset to the next sunrise). In the North Indian (purnimanta) calendar the same fortnight is counted in Bhadrapada.
About
Janmashtami falls on the eighth tithi of the dark half of Shravana in amanta reckoning, which North India names Bhadrapada Krishna Ashtami; it is the same night in August or early September. Because the birth is placed at midnight in Rohini nakshatra, the day whose nishitha (midnight) holds Ashtami is chosen, and the day on which Ashtami and Rohini both touch midnight is preferred. This is why smarta and Vaishnava calendars sometimes give consecutive days. Mathura, Vrindavan and Dwarka hold their main celebrations at the midnight hour.
How it is observed
A fast until midnight, the birth ceremony with the infant image at twelve, and dahi handi the next day in Maharashtra.