Go Tell It On the MountainBy Rabbi Jack Abramowitz
When Moshe was on Mt. Sinai, G-d gave him instructions pertaining to the Sabbatical year (Shemittah). When the Jews settle the land, they should work the land for six years, but in the seventh year they must let it rest. (This is analogous to the way the Jews themselves rest every seventh day for Shabbos.) In the Sabbatical year, crops may not be planted, worked or harvested, though what grows on its own may certainly be eaten.
Seven Sabbatical cycles of seven years each equals 49 years. The 50th year is the Jubilee (Yoveil in Hebrew). In that year, on Yom Kippur, the shofar is blown to declare liberty throughout the land. Slaves are released (regardless of the length of their service) and land returns to its hereditary owners. (Verse 10 is quoted on the Liberty Bell.)
As with the Sabbatical years, fields could not be worked on the Jubilee year. (This meant that there were two years in a row during which the land was not worked, the 49th and the 50th.)