What's the deal with Shavuot and Matan Torah?

(9) The LORD spoke to Moses, saying: (10) Speak to the Israelite people and say to them: When you enter the land that I am giving to you and you reap its harvest, you shall bring the first sheaf of your harvest to the priest. (11) He shall elevate the sheaf before the LORD for acceptance in your behalf; the priest shall elevate it on the day after the sabbath. (12) On the day that you elevate the sheaf, you shall offer as a burnt offering to the LORD a lamb of the first year without blemish. (13) The meal offering with it shall be two-tenths of a measure of choice flour with oil mixed in, an offering by fire of pleasing odor to the LORD; and the libation with it shall be of wine, a quarter of a hind. (14) Until that very day, until you have brought the offering of your God, you shall eat no bread or parched grain or fresh ears; it is a law for all time throughout the ages in all your settlements. (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 the LORD. (17) You shall bring from your settlements two loaves of bread as an elevation offering; each shall be made of two-tenths of a measure of choice flour, baked after leavening, as first fruits to the LORD. (18) With the bread you shall present, as burnt offerings to the LORD, seven yearling lambs without blemish, one bull of the herd, and two rams, with their meal offerings and libations, an offering by fire of pleasing odor to the LORD. (19) You shall also offer one he-goat as a sin offering and two yearling lambs as a sacrifice of well-being. (20) The priest shall elevate these—the two lambs—together with the bread of first fruits as an elevation offering before the LORD; they shall be holy to the LORD, for the priest. (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 the LORD am your God.

(1) 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. (2) And when they were departed from Rephidim, and were come to the wilderness of Sinai, they encamped in the wilderness; and there Israel encamped before the mount.

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 on the first day of the week He did not say anything to them due to the weariness caused by the journey. On the second day of the week, He said to them: “And you shall be to Me a kingdom of priests and a holy nation; these are the words that you shall speak to the children of Israel” (Exodus 19:6).

Rav Shemaya taught: Shavuot sometimes occurs on the fifth of Sivan, sometimes on the sixth of Sivan, and sometimes on the seventh. How so? If both the months of Nisan and Iyyar are full months of thirty days each, the festival of Shavuot, which is celebrated fifty days after the second day of Passover, occurs on the fifth of Sivan; if both months are short, with twenty-nine days each, it occurs on the seventh of Sivan; and if one of them is full and the other is short, it occurs on the sixth of Sivan. According to this opinion, if both months were full and the festival of Shavuot occurred on the fifth day of Sivan, and one made his vow on the day after Shavuot, i.e., the sixth of Sivan, and in the following year both months were short, so that the festival of Shavuot occurred on the seventh of Sivan, a whole year would have passed without three Festivals.

‘From the eighth of the same until the close of the Festival [of Passover], during which time the date for the Feast of Weeks was re-established, fasting is forbidden’. For the Boethusians held that the Feast of Weeks must always be on the day after the Sabbath. But R. Johanan b. Zakkai entered into discussion with them saying, ‘Fools that you are! whence do you derive it’? Not one of them was able to answer him, save one old man who commenced to babble and said, ‘Moses our teacher was a great lover of Israel, and knowing full well that the Feast of Weeks lasted only one day he therefore fixed it on the day after the Sabbath so that Israel might enjoy themselves for two successive days’. [R. Johanan b. Zakkai] then quoted to him the following verse, ‘It is eleven days’ journey from Horeb unto Kadesh-Barnea by the way of mount Seir. If Moses was a great lover of Israel, why then did he detain them in the wilderness for forty years’? ‘Master’, said the other, ‘is it thus that you would dismiss me’?

‘Fool’, he answered, ‘should not our perfect Torah be as convincing as your idle talk! Now one verse says. Ye shall number fifty days.1 while the other verse says, Seven weeks shall there be complete.How are they to be reconciled? The latter verse refers to the time when the [first day of the] Festival [of Passover] falls on the Sabbath, while the former to the time when the [first day of the] Festival falls on a weekday... R. Judah b. Bathyra says. There is written ‘Sabbath’ below and also ‘Sabbath’ above; just as in the former case the Festival, and indeed the beginning of the Festival, is near [to the Sabbath]. so in the latter case, too, the Festival, and indeed the beginning of the Festival, is near [to the ‘Omer]. Our Rabbis taught: And ye shall count unto you. that is, the counting is a duty upon every one. On the morrow after the Sabbath, that is, on the morrow after the Festival. Perhaps it is not so but rather on the morrow after the Sabbath of Creation. R. Jose b. Judah says, Scripture says, Ye shall number fifty days, that is, every time that you number it shall not be more than fifty days. But should you say that the verse refers to the morrow after the Sabbath of Creation, then it might sometimes come to fifty-one and sometimes to fifty-two and fifty-three and fifty-four and fifty-five and fifty-six. R. Judah b. Bathyra says. This is not necessary.