Matthew Poole and continuators on Jeremiah 36:9

9 And it came to pass in the fifth year of Jehoiachim, the son of Josiah king of Judah, in the ninth moneth, that they proclaimed a fast before the LORD to all the people in Jerusalem, and to all the people that came from the Cities of Judah unto Jerusalem.

This fast was appointed upon a particular emergency, whether it was for a Famine which was then in the Land, or to avert the ruine which they justly feared from the king of Babylon, who had lately brought them under his servitude is not certain; the yearly fast, Levit. 23.27. was to be kept in the seventh moneth, nor did God ever ordain any fast to be kept in the ninth moneth.