“Speak to the Bnei Yisrael and say to them,’When you come to the Land which I am giving you and the land will rest a Shabbos for Hashem…’” (25:2-3)
The terminology “Shabbos for Hashem” is similar to the term used for Shabbos Bereishis, the weekly Shabbos commemorating the Creation. What is the significance to this parallel?
There is a difference between Shabbos and Yom Tov. Shabbos comes every seven days and is not dependant upon human input. Yom Tov, however, is dependant upon the sanctification of Beis Din.
Shemitta and Yovel have a similar relationship. Every seven years (Shmitta) and every fifty years (Yovel) the land lays fallow. Additional laws of the Yovel are: blowing the shofar, freeing slaves, and returning ancestral lands to their original owners. However, these three laws and the law of not working the land in Yovel are interdependent; if these laws are not kept then the land does not lie fallow either. As opposed to Shemitta in which the land lies fallow in any case.
This is why it is called Shabbos for Hashem. Whether or not the years were counted or the laws observed, the produce becomes ownerless by the dictate of Hashem alone.
However with Yovel we find the expressions “And you will sanctify” and “For it is a Yovel, sanctified for you”. Clearly, yovel, while mandated by the Torah is, in some ways, controlled by people.
Shemitta is symbolic of the creation of the world (“for the land is Mine”) like Shabbos. But Yovel is connected to freeing slaves, reminiscent of leaving Mitzrayim, the basis of our celebrating Yom Tov.