Upon the fasting day — It was undoubtedly, because of the concourse of people which the prophet knew would that day be in the temple, that he chose that day, when some would be present from all parts of Judah.
Related Commentaries of Jeremiah 36:6
Jeremiah 36:6
6 Therefore go thou, and read in the roll, which thou hast written from my mouth, the words of the LORD in the ears of the people in the LORD'S house upon the fasting day: and also thou shalt read them in the ears of all Judah that come out of their cities.