R' Yehuda Amital:
The Magen Avraham questions why we celebrate Shavuot on the fiftieth day of the Omer, while in fact the Torah was given on Shabbat – which was the 51st day. The Maharal answers that what we are in fact celebrating is God's desire to give us the Torah, and to obligate us to receive it, even though perhaps Bnei Yisrael may have hesitated and faltered a little when they saw the great fire. When Bnei Yisrael were exposed to the great power of the Torah, it frightened them and caused them to have second thoughts about their commitment. Yet God nevertheless desired to give them the Torah, and even to force it upon them. It is God's readiness to give, and not Bnei Yisrael's readiness to receive, that we celebrate on Shavuot.
R' Aharon Lichtenstein:
What this teaches us is that after the sin [of the Golden Calf], the significance of matan Torah was greatly and seriously diminished. The nation had taken a step backwards, in a negative direction. We may compare the situation to that of a bride who was unfaithful under the very chuppa - it would have been better had she never entered the chuppa at all.
We originally posed the question of why the Torah hides the festival of matan Torah from us. Now the significance of the textual 'gap' is clear. Even Chazal refer to Shavuot as "zeman matan torateinu" (the time of the giving of the Torah) rather than as "zeman kabalat torateinu" (the time of the receiving of the Torah), because there was a giving of the Torah at the time, but not a proper receiving.
In summary: The Torah was given on what would have been the 50th day of the Omer, i.e. our holiday of Shavuot. It was only received by the Jewish people the next day, the day after Shavuot. This receiving was nullified (at least in our memories) by the sin of the Golden Calf. In its place, we try anew to receive and accept the Torah on each anniversary of the giving, which is to say each year on Shavuot.