From the Krishna Janmastami Wikipedia page:
Krishna Janmashtami, also known as Krishnashtami, Saatam Aatham, Gokulashtami, Ashtami Rohini, Srikrishna Jayanti, Sree Jayanti, or sometimes simply as Janmashtami, is an annual celebration of the birth of the Hindu deity Krishna, the eighth avatar of Vishnu.
The festival is celebrated on the eighth day (Ashtami) of the Krishna Paksha (dark fortnight) of the month of Bhadrapad (August–September) in the Hindu calendar. Rasa lila, dramatic enactments of the life of Krishna, are a special feature in regions of Mathura and Vrindavan, and regions following Vaishnavism in Manipur. While the Rasa lila re-creates the flirtatious aspects of Krishna's youthful days, the Dahi Handi celebrate God's playful and mischievous side, where teams of young men form human towers to reach a high-hanging pot of curd and break it. This tradition, also known as uriadi, is a major event in Tamil Nadu on Gokulashtami. Krishna Janmashtami is followed by the festival Nandotsav, which celebrates the occasion when Nanda Baba distributed gifts to the community in honour of the birth.
Hindus celebrate Janmashtami by fasting, worshipping Krishna and staying up until midnight, and offer prayers at special time when Krishna is believed to have been born. Images of Krishna's infancy are placed in swings and cradles in temples and homes. At midnight, devotees gather around for devotional songs, dance and exchange gifts. Some temples also conduct readings of the Hindu religious scripture Bhagavad Gita.