What was the sabbatical-year law that God gave Israel?
The sabbatical-year law required Israel to let the farmland rest every seventh year, leaving it fallow and not cropped. It was called a sabbatical year. God gave this law when Israel occupied Canaan. Failure to observe it caused the land to enjoy its Sabbaths during the seventy years of desolation Jeremiah foretold.
Les explains that when Israel entered the land under Joshua and occupied Canaan, God's law included a sabbatical year. Every seventh year the farmland was to be left fallow and was not to be cropped. Israel, however, had not been in the land very long before their greed got the best of them, and they failed to keep it. Therefore Jeremiah foretold that the land would enjoy her Sabbaths for as long as she lay desolate, keeping Sabbath to fulfill seventy years. This was fulfilled when Nebuchadnezzar carried away those who escaped the sword to Babylon, where they served him and his sons until the reign of the kingdom of Persia.