Shavuot is a principal example of a festival that was shaped by the victory of Rabbinic Judaism over other forms. It is first described, in the Torah, as the festival of the conclusion of the first fruits, brought to the Temple for the 49 days after Passover.
Later, it took on another character, as the festival of the giving of the Torah. Much like Shemini Atzeret/Simchat Torah, which became the festival of the Torah-reading cycle, Shavuot was made the festival of “Matan Torah” (the giving of the Torah, literally) as the Rabbis established their hegemony over Judaism and the Temple continued to lie in ruins and exile compromised the link between Judaism and the land of Israel.
Shavuot became a festival of revelation, then, and the accompanying rituals that formed around the holiday, such as the night vigil, the reading of Ezekiel 1 as the haftarah (another revelatory text) and the eating of light food in some communities combined to make the festival the gateway to summer. The summer is not, as I have written previously, a happy idyll in the Jewish experience, but punctuated by fasts, mourning and anxiety.
This is only our present phenomenology of the holiday, however. There have been many Judaisms and the original agricultural model lies under the sturm und drang, the grandeur and terror of the revelation that the Rabbis preferred to emphasize. What was once a guileless celebration of the first fruits, illustrated by the Mishnah’s lengthy description of the processional through Jerusalem to the Temple, turned into an edgy wait for the revelation at Sinai. This was acted out, in the popular rite, by a night vigil, in which sleepy devotees wreak an ordeal upon themselves, studying and drinking coffee and stumbling outside to contemplate the blackness of the night and the heaviness of the air. In Jewish neighborhoods around the world, shadowy figures move through the night. In Jerusalem, at the Western Wall, this will become a wave of humanity, the rough cacophony of voices coalescing, as the first rays of dawn break over the Mount of Olives, in the words of the Shema. Lacking a crack in the heavens and a Divine voice, this is the best revelation that we can produce.