Krishna Janmashtami, also called Gokulashtami, marks the birth of Krishna. Devotees keep a fast through the day, decorate a cradle, sing and recite the story of his birth in the prison of Kamsa, and break the fast at midnight, the hour he is said to have been born. The following day, in the Braj region and Maharashtra, young men form human pyramids to reach and break a suspended pot of curd, re-enacting Krishna's childhood raids on the household butter.
The festival falls on the eighth tithi (Ashtami) of the waning fortnight of the month in which the Rohini nakshatra is prominent, Bhadrapada in the Purnimanta calendar and Shravana in the Amanta calendar. Because the birth is tied to midnight, the fast and its breaking depend on local time.
Traditions weigh the factors differently: some observe the day when Ashtami covers midnight, others require the Rohini nakshatra as well, so smarta and Vaishnava dates can differ by a day.