Connection of Shavuot to Mt. Sinai

Question: How did the Rabbis of the Rabbinic period justified making a historical connection between the holiday of Shavu'ot and the giving of the 10 commandments at Mt Sinai/Horeb when ALL references in the Torah to Shavu'ot (Ex. 23:16, 34:22, Lev. 23:15-22, Num. 28:26-31, and Deut. 16:10,16) describe it only ...