The Supreme Personality of Godhead, Sri Krsna, advented Himself as Lord Caitanya Mahaprabhu, appearing in Sridhama Mayapura in the city of Navadvipa, Bengal in the year 1407 AD. Sri Krsna Caitanya manifested his divine mercy in Kali Yuga by delivering the Hare Krsna maha mantra and thereby saving the most fallen souls.
The divine appearance of Sri Krsna Caitanya Mahaprabhu is celebrated on the full moon of the month of Phalguna (February-March). This is the most auspicious day for all Gaudiya Vaisnavas, the most sacred and special festival to glorify the appearance in this world of the Supreme Personality of Godhead, Sri Krsna Caitanya. Sri Krsna Caitanya Mahaprabhu is none other than the Supreme Personality of Godhead Sri Krsna, who incarnates in the form of a devotee, to enjoy rasa. Srila Krsna dasa Kaviraja describes in the Sri Caitanya-caritamrta, that Sri Sri Radha and Krsna are one divine self, or one and the same, but they have taken two bodies. Both Radha and Krsna constantly enjoy each other, and taste the pleasures of divine love. To taste this divine mellow, Sri Sri Radha-Krsna have appeared in one body, as Sri Krsna Caitanya Mahaprabhu, who appeared in Mayapura Dhama.
It is explained in Srimad-Bhagavatam 1:3:28 that Lord Buddha appeared at the beginning of Kali Yuga as the 21st incarnation of the Lord. The final (22nd) incarnation described in that passage is Lord Kalki, who is predicted to appear at the end of Kali Yuga. While not described in this particular passage, elsewhere in Srimad-Bhagavatam Lord Caitainya Mahaprabhu is predicted to appear near the beginning of the Kali Yuga. In the seventh canto of Srimad-Bhagavatam, Prahlada Maharaja directly hints at the hidden nature of the Supreme Lord's appearance:
"Because the Supreme Lord is also called Tri-Yuga, or one who appears in only three yugas (satya, dvapara, treta), He is sometimes said to appear in a concealed form, in the age of Kali."
In the Atharva-Veda Purusa-Bodini-Upanisad, it is said: "In the seventh manvantara, in the beginning of the Kali-Yuga, the Supreme Personality of Godhead will, accompanied by His own associates, descend in a golden form to the earth. He will teach the chanting of His own names."
Hare Krsna Hare Krsna Krsna Krsna Hare Hare
Hare Rama Hare Rama Rama Rama Hare Hare
"Why did Krsna assume the form of Caitanya Mahaprabhu? It is explained that Krsna desired to know the glory of Radha's love. "Why is She so much in love with Me?" Krsna asked. "What is My special qualification that attracts Her so? And what is the actual way in which She loves Me?" It seems strange that Krsna, as the Supreme, should be attracted by anyone's love. We search after the love of a woman or a man because we are imperfect and lack something. The love of a woman, that potency and pleasure, is absent in man, and therefore a man wants a woman, but this is not the case with Krsna, who is full in Himself. Thus Krsna expressed surprise: "Why am I attracted by Radharani? And when Radharani feels My love, what is She actually feeling?" In order to taste the essence of that loving affair, Krsna appeared just as the moon appears on the horizon of the sea. Just as the moon was produced by the churning of the sea, by the churning of spiritual love affairs the moon of Caitanya Mahaprabhu appeared. Indeed, Lord Caitanya's complexion was golden, just like the moon. Although this is figurative language, it conveys the meaning behind the appearance of Caitanya Mahaprabhu."
Caitanya-caritamrta Adi-lila, Introduction
eko devah sarva-rupi mahatma
gauro rakta syamala-sveta-rupah
caitanyatma sa vai caitanya-saktir
bhaktakaro bhakti-do bhakh-vedyah
"The Supreme Personality of Godhead, who is the master of all transcendental potencies, and who may be known only by devotional service, [bhakti vedyah] appears in innumerable transcendental forms. He has appeared in red, white and black complexions, [gauro rakta syamala sveta rupah] and He will also appear in the golden form of Sri Caitanya Mahaprabhu. He will assume the role of a perfect devotee [bhakta akarah] and He will teach the conditioned souls the path of pure devotional service [bhakti dah]."
Sri Caitanya Upanisad, Text 6 (3)
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.