The day when father of Lord Chaitanya Mahaprabhu organized festivities in honor of Lord Chaitanya's birth.
The Festival of Jagannatha Mishra is an observance of the Jata karma samskara for baby Nimai (Shri Chaitanya Mahaprabhu). Following the fast for Gaura Purnima, which is broken upon the rising of the moon, the next day is held for feasting. On the Jagannatha Mishra festival day, the devotees meditate upon the Jata karman ceremony. The festival is also known as Anandotsava amongst the Gaudiya Matha devotees.
Srila Jagannath Mishra
"Jagannath Mishra was born in the village of South Dhaka in Sylhet (Sylhet). His grandfather, Madhu Mishra, had four sons: Upendra, Rangada, Kirtida, and Kirtivasa. His father and mother, Upendra Mishra and Kalavati, had seven sons: Kamsari, Paramananda, Padmanabha, Sarveshvara, Jagannath Mishra, Janardana and Trailokyanath." (Prema-vilasa, 24). According to the Gaura-ganoddesha-dipika, Upendra was Krishna’s grandfather, the cowherd Parjanya, while Jagannath was Nanda Maharaj in Krishna’s Vraja pastimes (Gaura-ganoddesha-dipika 37). Nanda or Jagannatha were, in Krishna’s various appearances, Kashyapa, Dasharath, Sutapa and Vasudeva.
vrindavane premarasakarau yau
babhuvatus tau na ca samshayo’tra
amu avishatam eva devavaditikashyapau
shrikaushalya-dasharathau tatha shriprishni-tatpati
"The king of Vraja has become Chaitanya’s father, Jagannath; the queen of Vraja is his mother Sachi. The son of Nanda is Chaitanya Gosai, and Baladeva is his brother Nityananda." (Chaitanya Charitamrita 1.17.294-295).