This fast was appointed upon a particular emergency, whether it was for a famine which was then in the land, or to avert the ruin which they justly feared from the king of Babylon, who had lately brought them under his servitude, is not certain; the yearly fast, Leviticus 23:27, was to be kept in the seventh month, nor did God ever ordain any fast to be kept in the ninth month.
Related Commentaries of Jeremiah 36:9
Jeremiah 36:9
9 And it came to pass in the fifth year of Jehoiakim the son of Josiah king of Judah, in the ninth month, 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.