Positive commandment · Mitzvat Aseh #140 of 613
To count the years of the Jubilee year and the shemitah years within it
למנות שני יובל שנים ושמיטים
The verse · Vayikra 25:8
וְסָפַרְתָּ לְךָ שֶׁבַע שַׁבְּתֹת שָׁנִים שֶׁבַע שָׁנִים שֶׁבַע פְּעָמִים וְהָיוּ לְךָ יְמֵי שֶׁבַע שַׁבְּתֹת הַשָּׁנִים תֵּשַׁע וְאַרְבָּעִים שָׁנָה
“And thou shalt number seven sabbaths of years to thee, seven times seven years; and the space of the seven sabbaths of years shall be to thee forty nine years.”
The Rambam’s words
To count the years of the Jubilee year and the shemitah years within it, as [Leviticus 25:8] states: "And you shall count seven shemitah years."
למנות שני יובל שנים ושמיטים, שנאמר: "וספרת לך, שבע . . ." (ויקרא כה,ח).
Why · Sefer HaChinuch #330
Counting the seven cycles of seven years
“It is from the roots of the commandment from the angle of the simple understanding that G-d, may He be blessed, wanted to inform His nation that everything is His; and in the end everything will return to those to whom He wanted to give it at first — for the earth is His, as it is written (Exodus 19:5), “for all the earth is Mine.””
Who & when
“The [laws of the sabbatical year] are practiced in the Land — and the release of monies in every place — from Torah writ.”
Where in the Mishneh Torah
Hilchot Sabbatical Year and the Jubilee הלכות שמיטה ויובל