The year of remission of debt was the seventh year of the calendar, not the seventh year of a specific transaction between people. All debts were cancelled, and Hebrew bond servants were released from their creditors (see 15:1–3).
Feast of Tabernacles: People celebrated this feast in the seventh month each year (Hebrew Tishri, September/October). It celebrated God's miraculous protection of Israel in the wilderness. It also celebrated the fall harvest (see 16:13–15).