Ancient Observance
The Torah tells us less about the first day of the seventh month than it does about festivals such as Passover or Sukkot.
The day was a sacred assembly. Ordinary work stopped. Trumpet or horn blasts marked the occasion, and special sacrifices were offered.
During the Temple period, the required offerings would have been presented at the Temple in Jerusalem. The sounding of horns also had a long place in Israel's worship and public life.
The biblical observance should not simply be pictured as the modern Rosh Hashanah celebration. The Torah does not mention apples and honey, the Days of Awe or many other customs now associated with the holiday.
Those features belong to the later development of the festival. The ancient foundation was a sacred day of rest, assembly, offerings and the sounding of the horn.