From The Solemn Assembly:

Not only were Solemn Assemblies a very common aspect in the revivals of the Bible, but they were a very important part of the life of believers in American during all its early years. For verification of this one has only to consult the Sprague Collection of Early American Pamphlets at the Weidener Library at Harvard university. There will be found a large number of sermons that were preached at the Fast Days and Solemn Assemblies which were frequently called and earnestly attended by American believers prior to the general decline of true Christianity which characterizes twentieth-century America.

Our Fathers believed God was offended by sin. They themselves were deeply troubled both by the existence of personal sin in their own lives and by the presence of unconfessed corporate sins in the churches and in the nation. They regarded natural calamities as manifestations of the displeasure of God Almighty against sin and allowed such events as earthquakes, fires, volcanoes, epidemics, floods, and droughts to prompt them to special seeking of God's face in fasting, prayer, and corporate repentance. They also sought the Lord in Solemn Assemblies in connection with wars, murders, rapes, etc., believing such outbursts of wickedness to be directly related to the general decline of moral and spiritual life in the churches.