Deuteronomy 31:10

מקץ שבע שנים AT THE END OF EVERY SEVEN YEARS [AT THE APPOINTED SEASON OF THE SHEMITTAH YEAR] — This means in the first year of the new Shemittah-period — in the eighth year (Sotah 41a). But why does Scripture call it the “Shemittah-year”, which name usually implies the seventh year? Because the law of the “seventh year” still applies to it, viz., in connection with the harvest of the seventh year’s produce that actually goes forth into (takes place in) the year following the seventh year (the eighth year) (Rosh Hashanah 12b; cf. Rashi on Exodus 34:21).

וַיְצַ֥ו מֹשֶׁ֖ה אוֹתָ֣ם לֵאמֹ֑ר מִקֵּ֣ץ ׀ שֶׁ֣בַע שָׁנִ֗ים בְּמֹעֵ֛ד שְׁנַ֥ת הַשְּׁמִטָּ֖ה בְּחַ֥ג הַסֻּכּֽוֹת׃

And Moshe commanded them, saying, At the end of every seven years, in the time of the year of release, in the feast of booths,