"O Herr, einer Deiner Namen lautet Triyuga,
The Supreme Lord Sri Krishna appeared as Sri Chaitanya Mahaprabhu
to establish Sankirtana Movement - the yuga dharma for this age of Kali. He appeared on Phalguni Purnima (the full moon day in the month of Phalguna) in the year 1486 AD (1407 Shakabda) at Sridham Mayapura as the son of Sri Jagannath Mishra and Srimati Sachidevi. His parents named him Nimai since the child was born under a nimba (neem) tree in the courtyard of His paternal house.
The Supreme Lord Sri Krishna wanted to personally inaugurate the sankirtana movement and teach us how to perform devotional service through His personal example. So He appeared as a devotee (bhakta rupa) accepting the mood of Radharani (the top-most devotee of the Lord). His appearance day is celebrated as Gaura Purnima.