On which day was the horn sounded throughout the land to begin the Year of Jubilee?

Answer: The Day of Atonement

Leviticus 25:9: the horn was sounded on the tenth day of the seventh month, the Day of Atonement, to begin the fiftieth year.

“Then you are to sound the horn far and wide on the tenth day of the seventh month, the Day of Atonement. You shall sound it throughout your land.”— Leviticus 25:9 (BSB)