דברים טו:א מדרש: Sifrei Devarim

(Devarim 15:1) "From the end (ketz) of seven years you shall make shemitah (a release)": I might think, from the beginning of the (seventh) year; we, therefore, reason: It is written here "ketz," and, elsewhere, (Ibid. 31:10) "ketz." Just as "ketz" there is at the end and not in the beginning, so, "ketz" here.