"You shall sound the shofar [ram's horn] on the tenth day of the seventh month, on Yom Kippur you shall sound the shofar throughout your land"Leviticus 25:9.

We are commanded to blow the shofar (ram's horn) on the tenth of Tishrei of the Jubilee Year—announcing the emancipation (without compensation) of all Hebrew slaves.

These shofar blasts parallel the annual sounding of the shofar on Rosh Hashanah, both with regards to the actual blasts as well as the blessings that accompany them. (Though the purpose of the sounding of the shofar is different; the Jubilee blasts serving as an announcement regarding the liberation of the slaves.)