Question
Why doesn't the Torah mention Rosh Ha-Shanah?
Answer
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.