Upon closer examination it may appear as though there may have been a distortion made by our esteemed sages and rabbis as to the meaning of the holiday. In reality, Shavuot is a national festival celebrating the offering up of the “bikkurim” at the Temple. The holiday is centered on ownership of land and nationhood...Such a claim can be made because while the other two of the shalosh regalim are mentioned in conjunction with both historical and agricultural events (Pesach - Ex. 13, Lev 23:10; Sukkot - Ex. 23:16, Lev. 23:39, 43), Shavuot mentions only an agricultural connection (Ex. 23:16, 34:22).
The sages and rabbis in their wisdom, not wanting the holiday to fall by the wayside attributed a new significance to Shavuot as a result of the new reality. Living in the Diaspora it became impossible to fulfill the biblical commandment of bringing the “bikkurim” to the Temple. Without the viability of the command, the holiday would have lost it purpose had the rabbis not made the new connection, the celebration of the giving of the Law.
Of course, the traditional point of view accepts unconditionally that the Sinaitic Revelation occurred on Shavuot. Rabbi Menachem Leibtag attempts to demonstrate how one can arrive at 6/7 Sivan for Ma'amad Har Sinai:
In the Mechilta (and in Mesechet Shabbat 86b), Chazal calculate that the Torah was given on either the sixth or seventh of Sivan (see also Rashi on [Exodus] 19:2->19), yet the fact remains that the Torah clearly prefers to obscure the precise date of this event. There is an additional manner by which it is possible to calculate the approximate date of Ma'amad Har Sinai. It is based on the assumption that the specific date of the tenth of Tishrei was chosen as 'Yom Kippur' because it marks the date when Moshe descended from Har Sinai with the second "luchot". If so, then we can calculate 'backwards', using the three sets of 'forty days' as described in the story of chet ha'egel in Devarim chapter 9; thus arriving at the following approximate dates: Forty days - second luchot: 1 Elul -> 10 Tishrei. Forty days - Moshe's prayer: 19 Tamuz -> 29 Av Forty days - first luchot: 6 or 7 Sivan -> 17 Tamuz.The main problem with such a calculation is that it relies on other assumptions (e.g., the Yom Kippur/2nd Tablets association) and not on any clearly stated textual chronology.
However we do have a very early source (300-400 years earlier than the Mechilta) that explicitly connects Shavuot with an historical event!
For those unacquainted with the fascinating Book of Jubilees, it is the earliest non-(Jewish)-canonical "Biblical book" extant, having been dated to approximately 150 BCE (examples being found among the Dead Sea Scrolls). Jubilees is also known as "Lesser Genesis", since it is a re-working of the books of Genesis and Exodus. Jubilees follows a tradition that is very strict in its approach to halacha. For example, 15:14 states: And the uncircumcised male who is not circumcised in the flesh of his foreskin on the eighth day, that soul shall be cut off from his people, for he has broken My covenant. Fortunately, the Pharasaic rabbis either mitigated the severity of an accepted halachic tradition or relied on a different one. But the important point here is that Jubilees records many early Jewish traditions, some of which are found in later midrashic sources (as well as some that are falsely ascribed as being Christian in origin, such as that of fallen angels!)
So what does Jubilees say about Shavuot? 6:15-17 states:
And He gave to Noah and his sons a sign that there should not again be a flood on the earth. He set His bow in the cloud for a sign of the eternal covenant that there should not again be a flood on the earth to destroy it all the days of the earth. For this reason it is ordained and written on the heavenly tables, that they should celebrate the feast of weeks in this month once a year, to renew the covenant every year.(Shavuot is again mentioned in chapter 22, but in connection with Abraham, Isaac, and Ishmael celebrating it as the feast of first fruits.)
So Jubilees associates Shavuot with a quasi-historical event - the covenantal renewal of God's promise not to destroy the earth! But this was a universal promise made to all of mankind rather than a unique covenant made exclusively with the Jewish people. It would surely pale in comparison with the Revelation at Sinai in the eyes of Jews in the 2nd century BCE. If there were a tradition connecting Shavuot to Sinai - THE seminal event of Judaism - surely the authors of Jubilees would have mentioned it.