Tonight Rosh HaShanah begins. It is probably safe to say that many Jews associate the holiday with two theological themes. The first, emphasized especially in children's books and early Jewish educational settings, is the birthday of the world - the day on which God created the world. The second theme, which is far more prominent in the liturgy, is that it is a day of judgment, the day on which God will determine "who will live and who will die... who [will die] by fire and who by water" as the prayer states. Rosh HaShanah opens not only the Ten Days of Repentence, but a longer period of atonement and judgment that traditionally ends about a month later with Hoshannah Rabbah, the last day of Sukkot (Tabernacles).
It is striking, though, that neither of these themes actually appear in the Bible. In fact, Rosh HaShanah is arguably the most underdetermined major holiday in the Bible. It is a day of "remembrance and trumpeting" (Leviticus 23:24). Remembrance of what? Why trumpet? When Ezra reads the Torah to the assembly in Jerusalem and introduces the people to Rosh HaShanah, he and his levitical assistants merely comment that it is day to celebrate (Nehemiah 8:9-12). Again, why they should celebrate is left obscure.
So this leads to a natural question, or better set of them: When, where, and why did Rosh HaShanah become take on these associations? Why did it become, specifically, the "day of judgment"?
To my knowledge, there are exceedingly few references to Rosh HaShanah in the literature of the Second Temple period. There may be a germ of these themes in Abraham's prayer found in Jubilees (12:16-21), a book written around 200 BCE. There Abraham mentions, on "the first day of the seventh month" God's creative power and asks for God's protection. These themes, though, are so common throughout this literature that it is hard to read much into them. There is also a very vague reference to the "day of remembrance" in the Dead Sea scrolls (4Q409 1.i.5).
Prior to the Rabbis, the first century Alexandrian Jew Philo provides the most extended discussion of this holiday. For Philo, the new year begins in Nisan and is commemorated with Passover. Our Rosh HaShanah he calls the "trumpet feast," and focuses on the meaning of the trumpet. He offers two explanations. One is that it is a remembrance of the giving of the divine law (thus solving the problem of what it is we should remember). Second is that it is a sign of war, both real war and the internal war that comprises the human condition (Special Laws, 2.188-192). He does not connect this holiday to Yom Kippur.
So what are we then to make the of the rabbinic understanding of Rosh HaShanah as a new year for judgment (Mishnah, Rosh HaShanah 1:1-2)? Where did this come from?
In my book Creating Judaism, I argued that sometimes rituals persist shorn of meaning; succeeding generations of Jews preserve the ritual but change the meaning. I think that Philo also points in this direction. Philo starts with the single ritual that is associated with Rosh HaShanah that is mentioned in the Bible, trumpeting. To him, trumpeting meant war. From that, the fuller meaning of the holiday followed.
I want to suggest - and this is admittedly pure speculation as I am writing this in order to procrastinate from my real work and don't have time to track this down - that the Rabbis had a different association with the trumpet (or shofar), and thus changed their understanding of the holiday accordingly. According to the Rabbis, the shofar is sounded at dangerous times to invoke God's judgment for good. Maybe the trumpet was by that time associated with civil legal proceedings; it certainly was associated with the Roman emperor. This new association of the shofar with judgment thus led to revaluing the holiday, and moving it toward a strong association with judgment, as it also got drawn into the strong gravitational force of Yom Kippur.
Perhaps a bit of stretch, but if you have a better idea I'd be delighted to hear it!