Albert Barnes Commentary Jeremiah 36:9

"Now it came to pass in the fifth year of Jehoiakim the son of Josiah, king of Judah, in the ninth month, that all the people in Jerusalem, and all the people that came from the cities of Judah unto Jerusalem, proclaimed a fast before Jehovah." — Jeremiah 36:9 (ASV)

The ninth month answers to our December, and the fast was probably in commemoration of the capture of Jerusalem by the Chaldaeans in the previous year.