The Jewish Calendar and Holidays (incl. Sabbath): Rosh HaShanah: Rosh Ha-Shanah in the Torah:
Why doesn't the Torah mention Rosh Ha-Shanah?
Probably the term Rosh Ha-Shanah isn't
used in the Torah because it would be ambiguous;
as the Mishnah (Rosh Ha-Shanah 1:1) tells us, there
are several different New Years. Celebrations on
the first day of the seventh month are mentioned in
Yechezkel 45:20 (according to the reading in the
Septuagint) and Nechemiah 7:72-8:12, but the term
Rosh ha-Shanah isn't used in those places either.
In spite of the lack of Biblical sources, it's
clear from the Mishnah that the first of Tishrei
was recognized from the earliest times as a major
New Year date. It undoubtedly became known very
early as "the" Rosh Ha-Shanah.
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