The 137th mitzvah is that we are commanded to blow shofar on the 10th of Tishrei of this [jubilee] year, and to declare throughout the land the emancipation of slaves and that each eved Ivri1 goes free on this day, the 10th of Tishrei, without payment.
The source of this commandment is G‑d's statement,2 "On the 10th day, Yom Kippur, of the seventh month, you shall make a proclamation with the ram's horn throughout your land" and3 "You shall declare the emancipation [of slaves] in the land to all its inhabitants."
It is explained4 that, "the jubilee is identical to Rosh Hashanah regarding blowing the shofar and the blessings." The laws of blowing the shofar on Rosh Hashanah are explained in tractate Rosh Hashanah.5
It is well-known6 that this shofar blowing on the jubilee is only to publicize the emancipation, and is part of the proclamation mandated by the verse "You shall declare the emancipation [of slaves] in the land." It is not like the shofar blowing of Rosh Hashanah, which is "a remembrance before G‑d," unlike this which is to designate the freeing of slaves, as explained above.
