At which feast was the law to be read to all Israel every seven years?

Answer: Feast of Tabernacles

At the end of every seven years, in the year of release, during the Feast of Tabernacles, the law was read in the hearing of all Israel.

“Then Moses commanded them, “At the end of every seven years, at the appointed time in the year of remission of debt, during the Feast of Tabernacles, when all Israel comes before the LORD your God at the place He will choose, you are to read this law in the hearing of all Israel.”— Deuteronomy 31:10-11 (BSB)