Fausset's Bible Dictionary
(See Exodus 23:10-11. Part of the same general law as the Sabbath day. The land must rest fallow each seventh year. In Leviticus 25:2-7 and Deuteronomy 15 God ordains also the release of debtors every seventh year. The parts of the harvest crop ungathered and ungleaned in some degree sowed themselves for a spontaneous growth in the idle seventh year (Leviticus 19:9; Leviticus 23:22). The owners laid up grain in the previous years for it Leviticus 25:20-22). As the Sabbath is God's assertion of His claim on time, so the sabbatical year on the land..)
The sabbatical year began in the seventh month, and the whole law was then read during the feast of tabernacles; so that holy occupation, not apathetic rest, characterized it, as in the case of the Sabbath day. At the completion of the week of sabbatical years, the Jubilee crowned the whole. Canaan's conquest took seven years, the allotment of land seven more; then began the law of the sabbatical year. These "years" were observed under the New Testament; and Judaizers even sought to force their observance on Gentile Christians (Galatians 4:10). In Luke 6:1 explain "the first Sabbath of a year that stood second in a sabbatical cycle." Josephus (Ant. 14:10, section 6) implies that at that time years were reckoned by their place in a sabbatical cycle. (See Ellicott, Life of Christ, p. 173-174, and note.)
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Fausset, Andrew R. Entry for 'Sabbatical Year'. Fausset's Bible Dictionary. https://www.studylight.org/dictionaries/eng/fbd/s/sabbatical-year.html. 1949.