leviticus 25:2 Speak unto the children of Israel, and say unto them, When ye come into the land which I give you, then shall the land keep a sabbath unto the LORD. - Free Bible Online

      2-4. When ye come into the land which I give you--It has been questioned on what year, after the occupation of Canaan, the sabbatic year began to be observed. Some think it was the seventh year after their entrance. But others, considering that as the first six years were spent in the conquest and division of the land (Jos 5:12), and that the sabbatical year was to be observed after six years of agriculture, maintain that the observance did not commence till the fourteenth year.
      the land keep a sabbath unto the Lord--This was a very peculiar arrangement. Not only all agricultural processes were to be intermitted every seventh year, but the cultivators had no right to the soil. It lay entirely fallow, and its spontaneous produce was the common property of the poor and the stranger, the cattle and game. This year of rest was to invigorate the productive powers of the land, as the weekly Sabbath was a refreshment to men and cattle. It commenced immediately after the feast of ingathering, and it was calculated to teach the people, in a remarkable manner, the reality of the presence and providential power of God.

JFB.