What was the "sabbatical year" and how was it observed in Old Testament times?
In Old Testament times, every seventh year was consecrated to the Lord. The land, inasmuch as it was the Lord's, was to "keep a Sabbath" unto Him. (Lev. 25:2-7.)
"In this year the self-sown produce of the arable lands was to be left for the poor and the beasts of the field; and the fruit of the unpruned vineyards and olive orchards was not to be harvested, but to be left to the owner, his family and servants, the stranger sojourning with him, his cattle, and the wild beasts. A release of debts owed by Israelite to Israelite was made.
"The year was intended to be not simply a year of leisure, but also one of religious instruction and exercises. To mark this, at the Feast of Tabernacles, (i.e., at the commencement of the sabbatical year, for it began with the seventh month) the whole law was read in the hearing of the people. The law of the sabbatical year was habitually broken by the Jews for a long period before the Babylonian exile. The 70 years of exile and the land's desolation were regarded as making up for the unobserved Sabbaths of the land. (2 Chr. 36:21.)"
Source: Bible Dictionary, 1979 LDS edition of King James Bible, p. 765.