Deuteronomy 15:1-18 talks about shemittat kesafim. In Deuteronomy 31:10-13, not unlike that day at Mount Sinai, the day is called yom haqhel.
The assembly is an emblem of covenant.
Shemitta is the year which is lo zarua, it is a midbar.
The haqhel returns the people to the desert where they were dependent on God, and to the moment of the covenant.