http://revach.net/article.php?id=4749

Section: Taryag Mitzvos   Category: Taamei Hamitzvos
Parshas Vayeilech: Kli Yakar - Why Is Hakhel On Succos Of Motza'ei Shmitta
The Kli Yakar says that Hakhel is meant to bring Shalom to Klal Yisroel as a prerequisite for tshuva. Tshuva is always more readily accepted when it come from a Tzibbur that lives in harmony and Achdus with each other.

The most auspicious time for this is right after Shmitta on Succos. The reason, says that Kli Yakar, is that the uneven distribution of wealth is the biggest source of friction among people. During Shmitta this factor is greatly minimized as no one owns the land and the poor and rich both eat off the land together without any lines demarcating ownership. Everything is Hefker to everyone on Shmitta.

Furthermore on Succos the rich leave their luxurious homes and together with the poor, every member of Klal Yisroel moves into a temporary hut, leveling the playing field in this all too competitive area. Succos is a time of true achdus, especially after Shmitta, and the perfect time for a dramatic call to tshuva and achdus by the King, the leader ofthe nation.