We might assume that the answer to this question is very simple: Shavuot ,uguca is on the 6th of Sivan iuhx because that’s what it says in the Torah – only it doesn’t.
It is true that Shavuot ,uguca is one of the five festivals introduced in the Torah (the other four are Pesach jxp, Rosh Hashanah vbav atr, Yom Kippur rupf ouh, and Sukkot ,ufx), and it is true that for four of the five festivals we are given a specific date to celebrate. Pesach jxp is the 15th day of the 1st month (Nissan ixhb), and in the 7th month (Tishrei hra,) Rosh Hashanah vbav atr is the 1st day, Yom Kippur rupf ouh is the 10th day, and Sukkot ,ufx is the 15th day. In contrast with these festivals for Shavuot ,uguca no date is given and instead we are told to count.
In Vayikra trehu (Leviticus) 23:15-16 it says: “And you shall count from the next day after the sabbath, from the day that you brought the sheaf of the wave offering; seven sabbaths shall be complete; to the next day after the seventh sabbath shall you count fifty days; and you shall offer a new meal offering to Adonai.”
While in Devarim ohrcs (Deuteronomy) 16:9-10 it says: “Seven weeks shall you count; begin to number the seven weeks from such time as you begin to put the sickle to the grain. And you shall keep the Feast of Weeks to Adonai your God.”
The challenge with the date of Shavuot ,uguca is that as you can see, the two passages contain different instructions. In Vayikra trehu the counting begins on the day after Shabbat (a Sunday) with a celebration on the 50th day, which would also be a Sunday. While in contrast in Devarim ohrcs the count is for seven weeks. This period of counting is today known as the Omer rng, and as you may know, we begin this count at the second day of Pesach jxp, not on a Sunday.
By fixing the count by date (beginning on the 16th of Nissan ixhb) instead of fixing it by day, the Rabbis ensured that Shavuot ,uguca would forever be on the same day in our calendar.
This might not seem like a very important fact to discuss but it was a vitally important decision in the establishment of rabbinic authority. The Torah, especially Vayikra trehu, appears clear about beginning the count on a Sunday – and in the Talmud we have evidence that those who read the Torah literally celebrated Shavuot ,uguca on a Sunday. But the Rabbis altered the understanding of Sabbath so that it was interpreted as meaning a festive day, and could be applied to the first day of Pesach jxp.
In changing the meaning of the Torah the Rabbis were able to establish their authority and to assert that their interpretations of Torah would take precedence, sometimes over the text itself. And in fixing the date of the festival they were also able to give Shavuot ,uguca significance outside of the Temple service, establishing it as the celebration of the date on which Torah was given. This served to wrestle power away from the Priesthood, arguably the primary rivals of the Rabbis for authority over the Jewish community.
In this way, when we come to celebrate Shavuot ,uguca this month we are not just celebrating a festival given to us in the Torah. We are not just celebrating the festival on which Torah was given to the Jewish people. We are also celebrating the festival which bears witness to the success of Rabbinic Judaism in establishing itself as the primary expression of Judaism in the aftermath of the destruction of the Second Temple, through until today.