Zman Matan Torateinu?

(9) You shall count off seven weeks; start to count the seven weeks when the sickle is first put to the standing grain. (10) Then you shall observe the Feast of Weeks for your God יקוק, offering your freewill contribution according as your God יקוק has blessed you. (11) You shall rejoice before your God יקוק with your son and daughter, your male and female slave, the [family of the] Levite in your communities, and the stranger, the fatherless, and the widow in your midst, at the place where your God יקוק will choose to establish the divine name.

(15) And from the day on which you bring the sheaf of elevation offering—the day after the sabbath—you shall count off seven weeks. They must be complete: (16) you must count until the day after the seventh week—fifty days; then you shall bring an offering of new grain to יקוק.

(21) On that same day you shall hold a celebration; it shall be a sacred occasion for you; you shall not work at your occupations. This is a law for all time in all your settlements, throughout the ages. (22) And when you reap the harvest of your land, you shall not reap all the way to the edges of your field, or gather the gleanings of your harvest; you shall leave them for the poor and the stranger: I יקוק am your God.

It is from the roots of the commandment from the angle of the simple understanding [that it is] since the entire essence of Israel is only the Torah, and because of the Torah were the heavens and earth created, and as it is stated (Jeremiah 33:25), “Were it not for my covenant day and night, etc.” And it is the essence and the reason that they were redeemed and left from Egypt — in order that they receive the Torah at Sinai, and fulfill it...

And because of this — that it is [the] essence of Israel, and because of it were they redeemed and went up to all of the greatness to which they rose — we were commanded to tally from the morrow of the holiday of Pesach until the day of the giving of the Torah; to show about ourselves the great desire [we have] for the honored day, which our hearts yearn [for] like “a slave seeks shade” and always tallies when will come the yearned-for time that he goes out to freedom...

IT IS ‘ATZERETH’...
By way of the Truth, [the mystic teachings of the Cabala], for in six days the Eternal made the heaven and the earth, and the seventh day is the Sabbath which has no partner, and the congregation of Israel is its partner, as it is said, and the earth, this being the eighth day. It is ‘Atzereth,’ for there everything is assembled. Now with respect to the Festival of Unleavened Bread He commanded that it be observed for seven days, with the first and seventh [days] being holy, though they are all holy and the Eternal is among them. From then on [i.e., beginning with the second day of Passover] we are to count forty-nine days[of the omer], which are seven weeks comparable to the [seven] “days” of the world, and then to sanctify the “eighth day” [i.e., the Festival of Weeks] just as the eighth day of Tabernacles [is holy]; and [the forty-nine days] counted between them are in the “intermediate days” of the festival, in the interval separating the first day and “eighth day” of the festival, this being the day of the Giving of the Torah when He made them to see His great fire and His words they heard out of the midst of the fire. Therefore our Rabbis, of blessed memory, always called the Festival of Weeks by the name of Atzereth [a name here used in the verse for the eighth day of the Festival of Tabernacles], for it is on “the eighth” day of the festival, which Scripture here so called by that name...

Rabbi Yehoshua ben Levi said: The assembly [atzeret] of the Festival [of Sukkot] should have been fifty days removed, corresponding to the assembly of Passover, but the assembly of the Festival, because [these days] transition from summer to winter, it would not be feasible for them to go and return at this time... That is: “How fair are your feet in sandals.”

The Sages taught: On the sixth day of the month of Sivan, the Ten Commandments were given to the Jewish people. Rabbi Yosei says: On the seventh day of the month. Rava said: Everyone agrees that the Jews came to the Sinai desert on the New Moon, as it is written here: “In the third month after the children of Israel were gone forth out of the land of Egypt, the same day came they into the wilderness of Sinai” (Exodus 19:1), without elaborating what day it was. And it is written there: “This month shall be to you the beginning of months; it shall be the first month of the year to you” (Exodus 12:2). Just as there, the term “this” is referring to the New Moon, so too, here the term is referring to the New Moon. And similarly, everyone agrees that the Torah was given to the Jewish people on Shabbat, as it is written here in the Ten Commandments: “Remember the Shabbat day to keep it holy” (Exodus 20:8), and it is written there: “And Moses said to the people: Remember this day, in which you came out from Egypt, out of the house of bondage, for by strength of hand the Lord brought you out from this place; there shall be no leaven eaten” (Exodus 13:3). Just as there, the mitzva of remembrance was commanded on the very day of the Exodus, so too, here the mitzva of remembrance was commanded on the very day of Shabbat. Where Rabbi Yosei and the Sages disagree is with regard to the determination of the month, meaning which day of the week was established as the New Moon. Rabbi Yosei held: The New Moon was established on the first day of the week...

...And the Rabbis hold: On the second day of the week the New Moon was established...

The Gemara raises an objection: Doesn’t the verse state: “And the Lord said to Moses: Go to the people and sanctify them today and tomorrow and let them wash their garments” (Exodus 19:10), indicating that the husbands and wives were separated for only two days? This is difficult according to the opinion of Rabbi Yosei, who said earlier that the separation was for three days.

The Gemara answers: Rabbi Yosei could have said to you: Moses added one day to the number of days that God commanded based on his own perception, as it was taught in a baraita: Moses did three things based on his own perception, and the Holy One, Blessed be He, agreed with him. He added one day to the days of separation before the revelation at Sinai based on his own perception. And he totally separated from his wife after the revelation at Sinai. And he broke the tablets following the sin of the Golden Calf.

I have a difficulty. How can we say on Shavuot that it is "the day of the giving of our Torah"?! Do we not conclude like R. Yose (Shabbat 86b) that the Torah was given on the 7th of Sivan? After all, we do conclude that separation from sexual intercourse [of man and wife, in regard to certain laws of Tumah] requires six half-days, as is written in Yore Deah 196. [R. Yose's opinion -- that the Torah was given on the 7th of Sivan -- is based on a need for husband and wife to separate a full three days. Since the Halakha today requires a 3 days for this particular separation, presumably R. Yose is correct about the date of Revelation.] (And I believe I've seen this question before in some Poseik's writings.) But in our calendar, Shavuot is always on the 6th of Sivan! In truth, the Rambam rules in the Laws of Avot haTumah Ch. 5, that we require only three half-days [of separation]. The Kesef Mishna comments there that in the Talmudic text of the Rambam and the Rashba, the Sages only require three half-days, and that the Rambam rules according to them. See there. If so, our custom is in line with the Rambam. What is written in Yoreh Deah, supporting R. Yose, is but stringency [chumra; it is the advised practice, but not the actual letter of the law]. A further difficulty: the Torah was given on the 51st day of their counting! For they left Egypt on the fifth of the week (Wednesday night / Thursday) and the Torah was given on a Shabbat. In this regard, the [book] HaAsarah Maamarot already answered, that it is meant to hint at the Second Day of Yom Tov of the Exiles. In fact, it’s possible that that’s the reason why Moshe added a day of his own accord, but this isn’t the place to discuss it