The appearance day of Lord Krishna, who the Vedic scriptures describe as the Supreme Lord, in this material world is celebrated as Janmashtami.
Some 5000 years ago, Krishna appeared on the midnight the eight day of waning moon (Ashtami tithi under Rohini Nakshatra) as the eight son of Devaki and Vasudev in Mathura district of Uttar Pradesh, India.
On this day devotees fast till mid-night and observe Janmashtami vrata and seek His blessings.
Bhagavad-gita mentions,
janma karma ca me divyam evam yo vetti tattvatah
The Lord’s appearance is not like that of an ordinary man, who is forced to accept a material body. He appears out of His own sweet will.
Lord Krishna is all-attractive, all-auspicious and all-blissful, and so are His pastimes, which are completely beyond the imperfections and contaminations of the material world and activities.