Shemitah and Yovel
There is a seven-year cycle and a related fifty-year cycle in the Jewish Calendar. The Shemitah Year, the Sabbatical Year, is each seventh year, and the Yovel, the Jubilee Year, is each fiftieth year.
The Shemitah Year, the seventh year, is analogous to the seventh day, the Shabbat, in that it is a "year of rest" for the Land. No planting or harvesting may be done that year; the population has to rely on the produce of the sixth year for three years, including the eighth, because no planting is permitted in the seventh, which the L-rd promises to supply with abundance (Leviticus 25, 20-21).
In Leviticus (25,20), we find "And if you ask, 'What will we eat in the seventh year; we haven't planted or harvested our grain?' " Hashem answers (Lev. 25,21) "I will command my blessing for you in the sixth year, and the Land will produce (sufficiently for) three years!"
Here too is an analogy to Shabbat, as celebrated initially by the Jewish People in the desert. There G-d provided for them on Friday, Erev Shabbat, a double portion (the origin of using "lechem mishne," a double portion of challah) of "manna," the miraculous food which the People found on the ground each morning, protected within layers of dew (the origin of the custom of covering the challot with a special challah cover), each day but on the Shabbat.
The Yovel is the fiftieth year; it occurs the year after seven cycles of seven years. With regard to agriculture, it is treated as a Shemitah year. Thus, the forty-eighth year must support, first of all, its own needs, plus those of the 49th, the 50th and the 51st years - a total of four years!
The Jubilee Year has some special laws:
1. All slaves were to be set free in the Yovel Year. A Biblical inscription to this effect was inscribed by the founding fathers of the United States of America on the Liberty Bell, which is now displayed in Philadelphia. The text reads, "And thou shalt proclaim liberty in the Land for all its inhabitants." (Lev. 25,10)
(Incidentally, many of the Founding Fathers were quite familiar with the Hebrew Bible and, according to one account, actually considered adopting Hebrew as the National Language. It is difficult to imagine the profound impact that such a decision would most probably have had on the national character!)
2. All sales of land were returned to the original owner in the 50th year. Thus, there were only leases of property for periods up to 49 years; there were no sales "in perpetuity" of parcels of land in the Land of Israel. "For the Land is Mine; you are only temporary residents and settlers together with me. (Lev. 25,23)"