Then Moses wrote this law and gave it to the priests, the sons of Levi, who carried the ark of the covenant of the Lord, and to all the elders of Israel. And Moses commanded them, “At the end of every seven years, at the set time in the year of release, at the Feast of Booths, when all Israel comes to appear before the Lord your God at the place that he will choose, you shall read this law before all Israel in their hearing. Assemble the people, men, women, and little ones, and the sojourner within your towns, that they may hear and learn to fear the Lord your God, and be careful to do all the words of this law, and that their children, who have not known it, may hear and learn to fear the Lord your God, as long as you live in the land that you are going over the Jordan to possess.” (Deut. 31:9-13 ESV)
Gideon hadn't started distributing pocket Torah scrolls so everybody could have his own, so they all came together once every seven years to hear it read. All of the Books of Moses were read during the 7 days of the Feast of Tabernacles. Later, when there were synagogues that had their own Torah scrolls, they would also hear portions of it read every week. That custom is referred to in Acts 15:21.
The TOG