The auspicious day of the appearance of Lord Krishna is celebrated all over the world as Sri Krishna Janmashtami. It is celebrated on the eighth day of the Krishna Paksha (waning moon) of the month of Shraavan. Krishna, who is the Supreme Personality of Godhead, appeared in Mathura as son of Vasudeva and Devaki.
“As stated in the Bhagavad-gita, the Lord says that His appearance, birth, and activities, are all transcendental, and one who understands them factually becomes immediately eligible to be transferred to the spiritual world. “ - Chapter 3: Birth of Lord Krishna,Krishna Book by His Divine Grace A.C. Bhaktivedanta Swami Prabhupada.
Lord Krishna Himself in Bhagavad gita, Chapter 4 text 8 describes the reason of His appearance as follows:
To deliver the pious and annihilate the miscreants, as well also restablish the principles of religion, I Myself appear millennium after millennium.