The fact that the Torah never refers to Rosh Hashanah as a day of judgment is fodder for much rabbinic interpretation. The two occurrences of the holiday in the Torah (Leviticus 23 and Numbers 29) state only that it is a holy day on which one refrains from work and on which there is the blowing of the shofar and the offering of various sacrifices. Neither source says anything about a "New Year", and certainly not a "Yom HaDin", a day of judgment. The former is asserted in the first Mishna (and associated gemara) of Tractate Rosh Hashanah and the latter in the second:
At four seasons [divine] judgment is passed on the world: at Passover in respect of produce; at Shavuot in respect of fruit; at New Year all creatures pass before him [God] like children of Maron, as it says, 'He that fashions the heart of them all, that considers all their doings'; and on Succot judgment is passed in respect of rain.
Nechemiah 8 gives us an interesting historical perspective; on the first day of the seventh month (i.e., Rosh Hashanah) Ezra reads and explains the "Law" before the congregation. The people weep, but then Ezra tells them "Go your way, eat the fat, and drink the sweet, and send portions unto him for whom nothing is prepared; for this day is holy unto our Lord; neither be ye grieved; for the joy of the LORD is your strength." And the people did so and made "great mirth"! Great mirth, on a day of judgment??
So what do other canonical and non-canonical books tell us regarding Rosh Hashanah - the Prophets, the Writings, apocryphal and pseudoepigraphical sources, the Dead Sea scrolls?
Nothing more is mentioned in any Biblical or extra-Biblical source. Was there some sort of great conspiracy to conceal the true nature of Rosh Hashanah?
What about the great Jewish historians Philo (20 BCE - 50 CE) or Josephus (37 CE – c. 100)? One would not expect them to be part of such a conspiracy. And both of them lived during the time of the Second Temple and were witness to all of the rituals of the holidays. Let's hear what they have to say.
Immediately after comes the festival of the sacred moon; in which it is the custom to play the trumpet in the temple at the same moment that the sacrifices are offered. From which practice this is called the true feast of trumpets, and there are two reasons for it, one peculiar to the nation, and the other common to all mankind. Peculiar to the nation, as being a commemoration of that most marvelous, wonderful, and miraculous event that took place when the holy oracles of the law were given; for then the voice of a trumpet sounded from heaven, which it is natural to suppose reached to the very extremities of the universe, so that so wondrous a sound attracted all who were present, making them consider, as it is probable, that such mighty events were signs betokening some great things to be accomplished. And what more great or more beneficial thing could come to men than laws affecting the whole race? And what was common to all mankind was this: the trumpet is the instrument of war, sounding both when commanding the charge and the retreat. There is also another kind of war, ordained of God, when nature is at variance with itself, its different parts attacking one another. And by both these kinds of war the things on earth are injured. They are injured by the enemies, by the cutting down of trees, and by conflagrations; and also by natural injuries, such as droughts, heavy rains, lightning from heaven, snow and cold; the usual harmony of the seasons of the year being transformed into a want of all concord. On this account it is that the law has given this festival the name of a warlike instrument, in order to show the proper gratitude to God as the giver of peace, who has abolished all seditions in cities, and in all parts of the universe, and has produced plenty and prosperity, not allowing a single spark that could tend to the destruction of the crops to be kindled into flame.According to Philo's understanding, Rosh Hashanah - as suggested by the shofar blasts - reflects 1) the giving of the Torah on Sinai to the Jews and 2) gratitude of all nations on earth to the giver of peace and prosperity.
How about Josephus?
But on the seventh month, which the Macedonians call Hyperberetaeus, they make an addition to those [Sabbath sacrifices] already mentioned, and sacrifice a bull, a ram, and seven lambs, and a kid of the goats, for sins.That's all you have to say about it, Joseph Ben Matisyahu??
Traditionalists believe that the Torah was written ca. 1200 BCE and academics somewhere in the 6th century BCE (or later; although even secular scholars do not preclude earlier sources for these traditions.) The Mishna reflects primarily post-destruction discussions and was finally redacted circa 200 CE. And only the Mishna - a relatively late source - equates Rosh Hashanah with a day of judgment. That's a gap up to 1000 years in which this central holiday of the Jewish calendar is never associated with judgment. How can this be? Why, on a day in which God supposedly determines the fate of every human being, would not this idea be emphasized and clarified?
The problem is further compounded by the fact that Yom Kippur, a holiday clearly stated by its very name as a day of atonement comes after Rosh Hashanah. Indeed, commentators have resorted to some very novel (and often convoluted) ideas to explain why there judgment comes before atonement.
So, yes, it does looks like what we have here is a conspiracy to conceal the true nature of this holiday! And that's exactly what Rabbi Yonatan Eybeschutz suggests in a quite forced (some would say silly) rationalization: that since gentiles believe in the Torah, explicitly stating that Rosh Hashanah is a day of judgment would cause them to repent more than the Jews, and this would result in a great accusation against the Jewish nation in heaven!
A modern teacher of the classical parshanim, Menachem Leibtag attempts to come to the rescue. Rosh Hashanah as a day of judgment can be derived from its description as Yom Teruah, a day of the shofar blast. In brief, the short oscillating sound of the teruah represented an imminent call to war (as opposed to the long blast of a tekiah signaling "all clear"). Thus the teruah of Rosh Hashanah creates "an atmosphere that simulates the tension and fear of war". Unfortunately, Rabbi Leibtag's explanation falls short of the mark - he is more making the case that the sound of the teruah is a preparatory warning - a call to teshuvah - for the upcoming holiday of Yom Kippur than a signal indicating the day of judgment has arrived.
So there is no escaping the fact that Rosh Hashanah has nothing to do with din. It appears that historically the New Year celebration was one of great joy and celebration (go argue with Ezra!) and that with time it transformed into a much more somber day (no sleeping on the day of judgment!), probably because rabbinic authorities felt that the explicitly solemn occasion of Yom Kippur required greater preparation.