Shemitas do not begin in the fall; they begin when Yehova’s years begin, at the new moon of the month of the Aviv barley, in the spring.
Fwiw, this is well outside my understanding.
A summary of I've learned since first hearing about the Shemitah year from Rabbi Cahn can be found here and is defined as:
"the seventh year in the seven-year agricultural cycle, when the land is left to lie fallow"That it begins in the fall, after the harvest, corresponding to celestial events, makes sense from an agricultural point of view.