In his Sefer Our Heritage, Eliyahu Kitov poses a puzzling question about the holiday of Shavuot. Originally, Matan Torah was scheduled for the sixth of Sivan, however, we see that Moshe delayed the giving of the Torah until the seventh. Furthermore, we know Bnai Yisrael counted fifty days in anticipation of the Torah, but if Moshe added an extra day, Bnai Yisrael would have counted fifty-one days. The most puzzling question raised is that if Moshe delayed the giving of the Torah by one day, why do Chazal say Matan Torah occurred on the sixth of Sivan? Furthermore, how can Moshe add a day without being told to do so by Hashem?
Eliyahu Kitov explains that the first time Moshe went to Bnai Yisrael, he told them that fifty days after they left Egypt they would receive the Torah. These fifty days were to be counted in full, so they counted in the morning, corresponding with the future Korban Omer. In the early days of the Midbar, day preceded night. This order is seen in Bereshit where it states that night proceeded day when discussing the sin of Adam and Chava (Bereshit 8:22).
Before Matan Torah, Bnai Yisrael were considered Bnai Noach and therefore the calendar day started with the daytime, just like the rest of the world counts. However, when Bnai Yisrael received the Torah they were restored to man's original state. Therefore, they counted the night before the day, as Hashem had originally intended. Hashem was going to give the Torah, and Bnai Yisrael were acting as witnesses to the covenant which would justify the world's continued existence. When Hashem asked Bnai Yisrael if they wanted the Torah, they answered, "All that God has said we will do..." In Moshe's mind, once Bnai Yisrael said this they had returned to man's original state and were now perfect. Consequently, they were worthy of counting night before day. The count that Bnai Yisrael had already begun was incorrect because they were counting day before night; therefore, they were missing a day.
Moshe reasoned that when Hashem told him, "And you shall sanctify this day and the morrow," since the day began without sanctification (because the previous night really should have started that day), that day should not be counted as one of the fifty, therefore the next night should mark the two full days of sanctification. Therefore, Hashem considered Bnai Yisrael's count of fifty days legitimate and counted the seventh of Sivan as the sixth. Shavuot is not really associated with a specific day of the month, rather it is a culmination of a fifty day counting period. Hashem considered the seventh an extension of the sixth, but now that we count based on the original calendar (night before day), we count fifty full days, until the sixth of Sivan.