There were numerous National Gatherings in the Old Testament which had ramifications for many years. For instance in 2 Chronicles 34 the nation followed God for a generation. In Nehemiah 8 when they gathered in the capital their repentance led to national reform and restitution of godly traditions.

Whether it was in ancient Judah or Israel or in gentile Nineveh, when the nation gathered, repented and worshipped God, great blessing always followed. So we can see that a gathering of a nation's people before God has deep roots and proven blessings in Biblical history. For example, at the dedication of the first temple under Solomon, the whole nation gathered in Jerusalem and offered sacrifices with singers leading great worship to the Lord (I Samuel 7:4-10). The result was God's Glory coming down on the temple.
We know that God will heal our land if His people humble themselves, seek his face and turn from their wicked ways (2nd Chronicles 7:14). In the time of Asa, the nation assembled in the capital, offered sacrifices, covenanted to seek and follow God and worshipped joyfully, with the result that God gave them rest on every side (2nd Chron 15:8-15).
In the days of Jehoshophat, they gathered, fasted, prayed, the prophetic word came, they fell down and worshipped. stood up and praised the Lord, so that they saw victory in battle and rest on all sides (2nd Chronicles 20:1-19). When Hezekiah was king, he called the people to the capital, though some of them mocked, some came. They prayed, offered sacrifices, rejoiced and
worshipped to see God answer their prayer and national reforms (2nd Chronicles 30:1-27).
During the reign of Josiah, the nation gathered and God's word was read out and the people covenanted to follow God so that the nation followed God for a generation (I St. Chronicles 34). In the book of Ezra we read (Ezra 10:1-17) that the nation gathered in the rain and the leaders prayed and wept and prostrated themselves before God due to the nation's faithfulness with the result that behaviour changed throughout the land.
Again, in the days of Nehemiah, the nation gathered In the capital and fasted in sackcloth and ashes, confessed their sins and their fathers' sins and had God's word read aloud for three hours, then confessed and worshipped God for three hours. The result was that the nation covenanted to follow the Lord.
But this national humbling followed by the great blessing of God was not confined to just ancient Israel and Judah! We know from the book of Jonah that the gentile city of Nineveh was saved from the severity of national judgement when the Ninevites responded to Jonah's call, fasting in sackcloth and ashes, believing in the true God and calling earnestly on him and turning from their old ways.