משך חכמה — מקץ שבע שנים במועד שנת השמטה בחג הסכות... הקהל

Q — Why is the mitzvah of Hakhel set specifically for Sukkot at the END of the שמיטה year?

A — After a full year barred from working the land, Israel are aching to return to plowing and sowing — material desire is at its peak. At THAT very moment they are gathered in the עזרה to hear devar Hashem before scattering back to the fields. The lesson is exactly the principle of the entire Torah: "להמתין עוד מעט לשמוע דבר ה'" — to subordinate gashmiyut to ruchaniyut at the moment the pull toward gashmiyut is strongest.