Five hundred years ago, there lived a great sage as well as spiritual master named Chaitanya Mahaprabhu, who was regarded as the founder of Gaudiya Vaishnavism. His followers were known as Gaudiya Vaishnavas.
Based on Hindu lunar calendar, Chaitanya Mahaprabhu was born on Phalguna Purnima in the year 1542 of Vikram Samvat. Accordingly, the followers of Chaitanya Mahaprabhu observe Phalguna Purnima as Gaura Purnima and observe the day as his birth anniversary.
According to Julian calendar, Chaitanya Mahaprabhu was born on February 18, 1486 C.E. Since the Gregorian calendar did not exist at the time of his birth, the proleptic Gregorian calendar calculated the birth date as Saturday, February 27, 1486 C.E.