Krishna Janmashtami (Sanskrit: कृष्णजन्माष्टमी, romanized: Kṛṣṇajanmāṣṭamī), also known simply as Krishnashtami, Janmashtami, or Gokulashtami, is an annual Hindu festival that celebrates the birth of Krishna, the eighth avatar of Vishnu. In certain Hindu texts, such as the Gita Govinda, Krishna has been identified as supreme God and the source of all avatars. Krishna’s birth is celebrated and observed on the eighth day (Ashtami) of the dark fortnight (Krishna Paksha) in Shravana Masa (according to the amanta tradition) or Bhadrapada Masa (according to the purnimanta tradition). This overlaps with August or September of the Gregorian calendar.

Source: https://en.wikipedia.org/wiki/Krishna_Janmashtami