The Torah commands a positive mitzvah that we make known to our offspring from generation to generation all that transpired there [Mt. Sinai], both visually and audibly. There is a very great purpose to this mitzvah. –Maimonides
You will not exactly find a verse in the Bible which says, “On Shavuot (Pentecost), God appeared to Israel at Mt. Sinai and spoke the Ten Words.” The belief that God gave the Torah to Israel on Shavuot is partly based on the account in Exodus and partly based on tradition.
We read in Exodus that Israel arrived at the foot of Sinai “on the third new moon [rosh chodesh]” (Exod. 19:1).
This does not mean, though I used to read it this way myself, that the Israelites were approaching three months of journeying since they left Egypt when they came to Sinai. Rather, it means on the first day of the third month of the year (Sivan), Israel arrived at Sinai. The first month is called Aviv (now called Nisan) and is the month of Passover. You may recall Israel left Egypt on the 15th, or midway through Nisan. The second month, Iyyar, had passed, and now Israel arrived on the first day of Sivan. They had been journeying approximately 45 days.
Shavuot occurs on the 6th of Sivan, five days after Israel arrived at Sinai. On Sivan 1, the people set up camp. On Sivan 2, Moses went up to talk to God. On Sivan 3, 4, and 5, the people sanctified themselves in preparation. And on Sivan 6, Moses went up again and God spoke to all Israel from the mountain.
This timeline is speculative and far from exact. It makes a number of assumptions that cannot be proven. It is, nonetheless, a reasonable guess.
More importantly, it follows a principle, that the great divine acts of redemption occur on the appointed times which God said the sun, moon, and stars point to in Genesis 1:14. The revelation of redemption through sacrificial blood came on Passover and the revelation of God’s will in the Torah came on Shavuot. There is a rightness to the notion that God follows such a pattern.
The sages, in fact, debated the issue of the timing the Sinai revelation in the Talmud (Shabbat 86 a-b). The majority of the sages argued that Israel arrived on Monday at Sinai, which was Rosh Chodesh, and Torah was given on Saturday which was both Shabbat and Shavuot. R. Yosei argued against the majority, however, and said Israel arrived on Sunday, and that Torah was not given until Saturday, which was in this case the day after Shavuot. The issue is not resolved and Yosei’s minority opinion remains as an alternative.
With all of this speculation and uncertainty about the timing of Torah at Sinai in relation to Shavuot, one thing is certain: God commands us to remember and pass on the story of Mt. Sinai in detail and with devotion. In the Maimonides (Rambam) citation at the top, we read a worthy description of this mitzvah. Where did Judaism come up with the idea that telling and passing on the story of Mt. Sinai is a commandment? It comes from Deuteronomy 4:9-10 (among other places):
Only take heed, and keep your soul diligently, lest you forget the things which your eyes have seen, and lest they depart from your heart all the days of your life; make them known to your children and your children’s children — how on the day that you stood before the Lord your God at Horeb [Sinai], the Lord said to me, ‘Gather the people to me, that I may let them hear my words, so that they may learn to fear me all the days that they live upon the earth, and that they may teach their children so.’
Let’s pass on the story, visually and audibly, as Israel received it at Sinai on a Shavuot long ago.