The advent of Lord Krishna is celebrated by devotees around the world with pomp and circumstance, a festival called Janmastami.
In the Bhagavad-gita, Lord Krishna states: "Whenever and wherever there is a decline in religious principles, and a predominant rise of irreligion, at that time I descend Myself." (Bhagavad-gita, Chapter 4, Text 7)
5,000 years ago demonic kings ruled the Earth and exploited her resources. At that time the Supreme Personality of Godhead, Sri Krishna, appeared in the land of Vrindavan. He performed godly feats, destroyed demons, lifted Govardhana Hill, and expanded Himself into hundreds of cowherd boys and calves to humble Lord Brahma. At the climax of His advent, documented in the epic Mahabharata as the battle of Kurushetra, Lord Krishna spoke the Bhagavad-gita to instruct mankind. Shortly thereafter He returned to His eternal abode, Goloka Vrindavan, in the spiritual sky.