“on a fast day.” The Jewish leaders would proclaim a fast day when there was some concern about a coming calamity and there was a need for prayer. By choosing a fast day to read the scroll, the people in the Temple would be concerned about a problem and in a mind to repent and confess sins, and he also knew there would be a large crowd in the Temple on a fast day, so lots of people would hear the Word of God being read. Thus a fast day would be a good chance for people to hear, believe, and repent.
Jeremiah dictated the scroll in the fourth year of Jehoiakim (Jer. 36:1), and an appropriate fast day came up the next year, the fifth year of Jehoiakim (Jer. 36:9). We do not know when in the fourth year Jeremiah wrote the scroll, but we do know that Baruch read it in the ninth month of the fifth year, which is our November/December (Jer. 36:22). This means there was at least a nine-month wait between when the scroll was written and when it was read. This is a great lesson in patience. Even though Jeremiah and Baruch wanted to read the Word of God to the people, they waited for the time that God told them to wait for. Spiritual maturity involves learning to wait for the right time to do God’s work. Although some commentators believe this fast day to be the Day of Atonement, that cannot be correct because this fast occurred in the ninth month (Jer. 36:9), while the Day of Atonement was the tenth day of the seventh month (Lev. 16:29).
“in the house of Yahweh.” The house of Yahweh is the Temple. Baruch was to read Jeremiah’s prophecy in the Temple to the people there.