The Torah book containing the command for a first-day seventh-month memorial with horn blasts is Leviticus.
The passage appears in Leviticus 23:23–25, within the chapter’s calendar of sacred occasions. It specifies the first day of the seventh month as a day of rest and a holy convocation, marked by a memorial proclaimed with blasts. In the Hebrew calendar, this observance became associated with Rosh Hashanah.
Leviticus is the third book of the Torah, following Exodus and preceding Numbers. Much of the book concerns priestly duties, sacrifices, ritual purity, holiness, and the annual festival calendar. Chapter 23 is especially important because it lists the appointed festivals and sacred days, including Passover, Shavuot, Yom Kippur, and Sukkot.
The passage itself describes the day and its blasts but does not provide the later liturgical name Rosh Hashanah in the same form. That name, meaning “head of the year,” became standard in Jewish tradition. The shofar ceremony and the detailed patterns of its blasts were developed through subsequent Jewish interpretation and practice.