Note: I did the translations and transliterations (Aside from the long passage from BT Shabbos, which is Soncino) and take responsibility for any errors.
There are two holidays which we celebrate in a manner distinctly different from their biblical descriptions: Simchas Torah and Shavuos
Both holidays are tied to the Torah. Simchas Torah because we finish an annual cycle, and Shavuos due to the tradition that Matan Torah/The Giving of the Torah, occurred on the sixth day of Sivan
However, they differ because the customs of Simchas Torah are a customary evolution, not found in the primary rabbinic sources. The customs had already begun by the Geonic period, when R” Hai Gaon wrote extensively condemning the practices of Simchas Torah. The justification for these customs is found in the מהרי"ק as cited by the רמ"א permitting at least the custom to dance since (1) one will not come to repair musical instruments, a labor forbidden on shabbos, if the dancing is for the honor of the Torah (a סברא/reasoning which contradicts the explicit prohibition on BT Betzah 36b), and (2) because “מנהג דוחה הלכה”/ "a custom overrides a law" an unprecedented statement with no earlier textual basis, often cited by the Tosafists as the basis for ruling against Talmudic law.
But Shavuos is different. Thought the Torah mentions neither the date of Shavuos nor that it falls out on the date of Matan Torah/The Giving of the Torah, there is a midrashic precedent for this idea in Mechilta d'Rabbi Yismael (a collection of halachic midrash on Exodus)
The problem with this interpretation arises from BT Shabbos 86b where a dispute is recorded between R. Jose and the Sages as to the date of Matan Torah/The Giving of the Torah. All agree that Jewish people arrived at Sinai on the first day of Sivan. All agree that the Torah was given on Shabbos (both are derived by gezerah shavah). However R. Jose rules that that the first day of Sivan was on Sunday, and the Sages say it was Monday:
ת"ר בששי בחדש ניתנו עשרת הדברות לישראל רבי יוסי אומר בשבעה בו אמר רבא דכולי עלמא בר"ח אתו למדבר סיני כתיב הכא (שמות יט, א) ביום הזה באו מדבר סיני וכתיב התם (שמות יב, ב) החדש הזה לכם ראש חדשים מה להלן ר"ח אף כאן ר"ח ודכולי עלמא בשבת ניתנה תורה לישראל כתיב הכא (שמות כ, ז) זכור את יום השבת לקדשו וכתיב התם (שמות יג, ג) ויאמר משה אל העם זכור את היום הזה מה להלן בעצומו של יום אף כאן בעצומו של יום כי פליגי בקביעא דירחא רבי יוסי סבר בחד בשבא איקבע ירחא ובחד בשבא לא אמר להו ולא מידי משום חולשא דאורחא בתרי בשבא אמר להו (שמות יט, ו) ואתם תהיו לי ממלכת כהנים בתלתא אמר להו מצות הגבלה בארבעה עבוד פרישה ורבנן סברי בתרי בשבא איקבע ירחא בתרי בשבא לא אמר להו ולא מידי משום חולשא דאורחא בתלתא אמר להו ואתם תהיו לי בארבעה אמר להו מצות הגבלה בה' עבוד פרישהAs a general principle, when an unresolved Talmudic dispute is recorded between an individual and the Sages, we rule according to the majority, ie the Sages. However on BT Gitten 67a we are taught an explicit exception for R. Jose, who is followed over the Sages, because "נימוקו עמו" or “his (superior) logic is with him.”
On the sixth day of the month [Siwan] were the Ten Commandments given to Israel. R. Jose maintained: On the seventh thereof. Said Raba: All agree that they arrived in the Wilderness of Sinai on the first of the month. [For] here it is written, on this day they came into the wilderness of Sinai; whilst elsewhere it is written, This month shall be unto you the beginning of months: just as there the first of the month, so here [too] the first of the month [is meant]. Again, all agree that the Torah was given to Israel on the Sabbath. [For] here it is written, Remember the Sabbath day, to keep it holy; whilst elsewhere it is written, And Moses said unto the people, Remember this day: just as there, [he spoke] on that very day, so here too it was on that very day. [Where] they differ is on the fixing of the New Moon. R. Jose holds that New Moon was fixed on the first day of the week [Sunday], and on that day he [Moses] said nothing to them on account of their exhaustion from the Journey. On Monday he said to them, and ye shall be unto me a kingdom of priests Tuesday he informed them of the order to set boundaries, and on Wednesday they separated themselves [from their wives]. But the Rabbis hold: New Moon was fixed on Monday, and on that day he said nothing to them on account of their exhaustion from the journey. On Tuesday he said to them, and ye shall be unto me a kingdom of priests; on Wednesday he informed them of the order to set boundaries, and on Thursday they separated themselves.
How then do we rule like Sages in the case of Shavuos?
The simple answer is that we do not. The halacha is according to R. Jose that the Torah was given on 7 Sivan, and that as a result we celebrate the Giving of the Torah on Shavuos (in accordance with the Mchilta). However, Shavuos does not have a fixed date. It falls out 50 days after the Korban Haomer is brought, which in the fixed calendar always coincides with 6 Sivan. Prior to the fixed calendar, Shavuos could fall on 5 Sivan (If Iyar and Sivan were 29 day months), on 6 Sivan (If Iyar and Sivan were split, with one a 29 day month and the other a 30 day month) or on 7 Sivan (If Iyar and Sivan were both 30 day months.)
The Giving of the Torah is tied to Shavuos because it would naturally fall out on the “correct” date roughly one third of the time. We rule like R. Jose and thanks to the set calendar we always celebrate the Giving of the Torah on Shavuos, which falls out on 6 Sivan the wrong day for the Giving of the Torah
Search for more information about the Talmud at 4torah.com.
Going to Israel?
Now get 2 phones for the price of 1 (and free calls too) with Talk'n'Save.