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4. When was the first Shemita observed?
The counting of Shemita first began when the Jews entered Eretz Yisrael and settled the lands. This occurred 14 years after the entrance into Eretz Yisrael, as it took 7 years to conquest the land and another 7 to settle it. Hence, the first Shemita was only observed 21 years after entering Eretz Yisrael.
 Rambam Shemita 10:2; Toras Kohanim Behar 1; Aruch Hashulchan Haasid 15:2; Encyclopedia Talmudit Vol. 2 p. 220