Shabbat 87 - When was the Torah given?
The Torah was given on the sixth of the month Sivan; this, therefore, is when Shavuot, marking the giving of the Torah, is celebrated. However, Rabbi Yose says that this happened on the seventh of Sivan.
Rava put their disagreement in perspective: all agree that the Jews arrived to Sinai on the first of the month. How do we know? Because in Egypt God said, "This month (Nissan) will be for you the first of the months," on at Sinai it says "On this day they arrived." The word "this" connects the two and teaches us that both were the first of the month.
Furthermore, all agree that the Torah was given on Shabbat. And why is that? - Because part of the giving of the Torah was "Remember Shabbat," so this must have happened on Shabbat itself, just as in Egypt, where it says "Remember this day," and it was said on this very day.
If so, where do they disagree? - They disagree about on what day was the first of the month. Then what does Rabbi Yose do with the extra day in his count? - He says that Moses added an extra day of preparation, but God later agreed with him. Actually, Moses did three things on his own, to all of which God later agreed. The first is the one we just mentioned, the second is when he separated from his wife, so that he could be ready to experience Divine presence at any time, and third is when he broke the tablets. How do we know that God agreed? - From the last words of the Torah, "Which Moses broke in front of everyone." God said, "Well done!" with no trace of displeasure.
Art: Claude Vignon - Moses with the Tablets of the Law