It is celebrated on the next day of Janmashtami. This festival attracts thousands of enthusiastic people from all over Maharashtra to Mumbai and Pune where the celebrations reach their peeks.
Nowadays Dahi Handi or Gopalkala has become compulsory ritual in Janmashtami celebrations. We can understand the importance and social significance of these festivals as that there are some professional groups involved with the celebrations in Mumbai and Pune.
Nandotsav or Nanda Mahotsav is celebrated in many temples dedicated to Lord Sri Krishna on the day next to Janmashtami.
Check this video for detailed Janmashtami Puja Procedure with Mantras and fully explained Puja steps..
Gopalkala 2013 or Dahi Handi 2013 date was August 29.
Gopalkala 2012 or Dahi Handi 2012 date was August 10.