The גמרא in Berachos 35b relates that רבא would tell his students not to appear before him during the months of ניסן ותשרי because of the work that they needed to do in the fields. Thus, two months of each year being בטלים בתורה, after six years there were twelve months of learning that needed to be made up. Accordingly, the seventh year is שנת שבתון.
05/10/2021