וְהַֽעֲבַרְתָּ֞ שׁוֹפַ֤ר תְּרוּעָה֙ בַּחֹ֣דֶשׁ הַשְּׁבִעִ֔י בֶּעָשׂ֖וֹר לַחֹ֑דֶשׁ בְּיוֹם֙ הַכִּפֻּרִ֔ים תַּעֲבִ֥ירוּ שׁוֹפָ֖ר בְּכָל־אַרְצְכֶֽם׃
Then shalt thou make proclamation with the blast of the horn on the tenth day of the seventh month; in the day of atonement shall ye make proclamation with the horn throughout all your land.
Read kabbalistic (mystical) interpretation of Leviticus 25:9: and classical mystical sources.