We find three names in the Torah for Shavuot. In the chapter discussing the festivals (Leviticus 23) no name is mentioned at all. Instead, the Scripture refers to it in connection with the commandment of counting sefirah (Id. v. 16): "to the next day after the seventh sabbath shall you count fifty days; and you shall offer a new meal offering to the Lord" (ad ha-shabbat ha-sh'vi'it tisp'ru hamishim yom v'hiqravtem minhah hadashah la-Sheim." It is also written (Id. v. 21): "it shall be a statute forever in all your dwellings throughout your generations" (huqat olam l'doroteikhem b'khol moshvoteikhem). The plain meaning of the Scripture here is that we should not mistakenly say that the festival that is stems from the bringing of the Omer differs from the other festivals in being dependent on residence in the land of Israel, in which case it would not have been observed outside of Israel. The Torah therefore emphasized that it is a statute forever - even when there is no Holy Temple and even outside the land of Israel. However in poroshat Mishpatim (Exodus 23:16) this festival is referred to as "the Feast of Harvest, the first fruits of your labors" (hag ha-qatzir bikurei ma'asekha). Since the Scripture had already referred to it as Hag Ha-Qatzir, it was therefore appropriate that the Torah emphasized that it is an everlasting statute in order to preclude any mistaken belief that this holiday is one of those commandments that is dependent upon the land.
In Poroshat Ki Tisa (Exodus 34:22) and in the enumeration of the offerings in poroshat Pinhas (Numbers 28:26) we encounter a new name, "Shavuot," denoting the seven weeks that we count from the day of the bringing of the Omer on the second day of Passover. And thus is its name established in poroshat R'eih (Deuteronomy 16:10): "and you shall keep the Feast of Weeks to the Lord your G-d" (v'asita hag shavu'ot la-Sheim Eloqekha). In poroshat Pinhas (Numbers 28:27), we find the third name - Yom Ha-Bikurim: "Also in the day of the firstfruits, when you bring a new meal offering to the Lord, in your feast of weeks" (u'v'yom ha-bikurim, b'haqrivkhem minhah hadashah la-Sheim, b'shavu'oteikhem). The Torah says in your feast of weeks (b'shavu'oteikhem) to emphasize, contrary to the Sadducees (who held based on the Scripture "to the next day after the Sabbath you shall count fifty days" that Shavuot must fall on Sunday), that it is "your feast of weeks" (shavu'oteikhem). This teaches us that the Shavuot is determined by us through the Jewish court's power to declare the beginning of the new month. Since the day on which Passover is determined by us, the subsequent occurrence of Shavuot is also determined by us. But, according to the opinion of the Sadducees, that counting begins on Sunday, the occurrence of Shavuot would not depend on any human decision.
However, in many places in the Mishnah we encounter a new name, "Atzeret," a name that we come across in the Torah in the chapter of the festivals in reference to the eighth day of Sukkot, which is called "Atzeret." The eighth day, as well, is called "Atzeret" in poroshat Pinhas (Numbers 29:35): "On the eighth day you shall have a solemn assembly" (ba-yom ha-sh'mini atzeret tih'yeh lakhem). In poroshat R'eih (Deuteronomy 16:8) the seventh day of Passover is also called "Atzeret." But in the Mishnah the term "Atzeret" is reserved exclusively for Shavuot. This terminology is interesting, and it requires, of course, a simple explanation. Some of what I shall say is already found in the works of other authors, I have therefore come only to add a bit of water and flour.
Now all three names in the Torah "Hag Ha-Qatzir" "Yom Ha-Bikurim" and "Hag Shavuot" are easily understood, and despite their differences, signify but one thing. After the sprouting and ripening of the wheat and the fruit of the trees, the Torah commands us to celebrate the blessing of the land and that we have merited the kindnesses of the Eternal who is constantly focused on the land of Israel. It is this concept that is signified by two of the names "Hag Ha-Qatzir" and "Yom Ha-Bikurim." The third name "Hag Shavuot" expresses the connection between the harvest of the wheat and the reaping of the Omer, the first sheaf of our crop.
Let us now try to understand why the Sages of the Mishnah saw fit to substitute a new name, "Atzeret," for Shavuot. What does this name, a name borrowed from the seventh day of Passover and the eighth day of Sukkot, express in relation to Shavuot?
When we reflect and arrive at the Divine secret why these two days in particular were called "Atzeret" in our holy Torah, we will necessarily understand the depth of the intention of our Sages in applying the name "Atzeret" to Shavuot.
Now the reason for why the seventh day of Passover, on which it is prohibited to do creative work and is included in the category of "holy assemblies" (miqra'ei qodesh), is a Holy Day is nowhere given explicitly. But how correct and true are the words of the S'forno in poroshat R'eih, which I have already quoted in my essay on Passover, that the great miracle of the splitting of the Red Sea occurred on the seventh day of Passover. So it was to commemorate this event that the seventh day of Passover was made a Holy Day and a Festival, which is why the Song of Moses is read on this day. There is no specific commandment, other than the prohibition to do work, that clearly provides any commemorative symbol for this Holy Day and Festival. The prohibition against eating hametz is of course a purely negative activity and applies on the intermediate days as well, while eating matzah is an obligatory commandment only on the first night of Passover. The Torah calls the seventh day, in contrast to the intermediate days, "Atzeret" to signify that its main characteristic is the cessation of work and that we are confined to our homes but may not work in the field. Exactly the same is true of the eighth day of Sukkot. The commandments of the sukkah and the lulav have lapsed, and all that remains is the prohibition to do work. The Torah therefore also calls this day "Atzeret." (In my essay on Shemini Atzeret you will find a comprehensive explanation.) So it is clear that the seventh day of Passover and the eighth day of Sukkot, which have no specific commandment and no visible symbolic content, were necessarily called "Atzeret" to signify the prohibition of doing work. But Shavuot, the day on which the Sh'tei ha-Lehem and the Bikurim were brought, did have an external symbol perceptible to our senses, and was given the names "Hag Shavuot" "Hag Ha-Qatzir", and "Yom Ha-Bikurim" in recognition of those symbols.
But this made sense only before the destruction of the Temple. After the destruction, after the the Omer could not be reaped, the wheat could not be harvested, and the bikurim could not be brought, our Sages were compelled to find and to coin a new name that was appropriate to the painful condition of exile and to the loss of the essential character of Shavuot and the harvest of the wheat. Now it is true that the name "Shavuot" signifies the counting of seven weeks and the occurrence of Shavuot on the fiftieth day. And we fulfill this commandment in our days as well. So it would appear that our Sages should have upheld the name "Shavuot." Why then did our Sages find it necessary to seek after a new name for the festival that was not mentioned at all in the Torah itself?
That the Sages of the Mishnah did not maintain the name "Shavuot" after the destruction provides compelling proof to the opinion of the Tosafot in Menahot 66a that the counting of the Omer in our days is but a rabbinic obligation, which is intended as a commemoration of the temple times (zeikher l'miqdash). This is an extraordinary proof to the opinion of the Tosafot.
However, our question then becomes even greater according to the opinion of the Rambam who rules in Hilkhot t'midin u-musaphin that the counting of the Omer even in our days is Biblically mandated. Why, according to the Rambam, did our Sages substitute a new name, "Atzeret," for the Biblical name "Shavuot" which represents the counting of seven weeks as it is explicitly written in the chapter of the Festivals and in the poroshat R'eih? This is a powerful question against the opinion of the Rambam, and I have not seen that any of the commentators were provoked by it.
And by pure chance the extraordinary work Hazon Yehezqeil on the Tosefta came into my hands, and in the first chapter of the tractate Rosh Hashanah he touches on this subject in his commentary, but he saw it only in part but not in its entirety. For, even after his explanation, the question why this Holy Day is called "Atzeret" rather than "Shavuot" in the Mishnah remains unanswered. And I have also seen that Rabbi Zevin, in his lovely work, Ha-Mo'adim B'Halakha, brings the Targum to the verse "Also in the day of the firstfruits, when you bring a new meal offering to the Lord, in your feast of weeks" (u-va-yom ha-bikuirm b'haviakhem minhah hadashah la-Sheim b'shavu'oteikhem) which translates "shavu'oteikhem" as "atzroteikhon" (your cessation of work), which corresponds to the usage of the Mishnah. And the question was also raised in the Pesiqta Zutarta.([Midrash Lekah Tov].
R. Toviah b. R. Eliezer said, "I have reviewed all the material concerning the Festivals and I have not found that Shavuot is anywhere called "Atzeret." But our Rabbis, of blessed memory, have in every instance called Shavout "Atzeret."
In the Pesiqta Zutarta, this question is left as an unsolved puzzle.
And I now come to add something to the words of these great authors, for it is possible to bring a compelling proof that in the time of the Mishnah, no association was made between Shavuot and the great event of the giving of the Torah. It is only later in the Gemara (Pesahim 68) where it is said in the name of R. Elazar that everyone agrees that on Atzeret part of the day must be devoted to one's own personal enjoyment (hetzyo lakhem) inasmuch as it is the day on which the Torah was given. But in the Mishnah itself there is no mention of the giving of the Torah on Shavuot. Moreover, it is said in the Mishnah (Megilah 30b) that on Atzeret the Torah portion that is read is "seven weeks" from poroshat R'eih (as opposed to the portion recounting the giving of the Torah). This reinforces our question how, according to the opinion of the Rambam that there is a Biblical obligation to count seven weeks even in our own days, the Sages could have abandoned the name "Shavuot" that was established by the Torah.
But after much effort and far-flung research I found that it is written in the Siddur Iyun Tephilah that the Sages changed the name Shavuot in order not to provide credence to the Sadducees who assert that the verse "on the next day after the Sabbath" (mi-mohorat ha-shabbat) refers to the Sabbath of Creation (Saturday) not, as the Rabbis held, the first day of Passover. It thus appears that the Rambam concurs with the Pesiqta Zutarta, which is also cited by the Iyun Tefilah in the Kiddush for festivals in the name of Rabbeinu Toviah:
Why is it said concerning Atzeret and Yom Ha-Kippurim "one goat" (s'ir izim ehad) instead of "and one goat" (u-s'ir izim ehad) as it is written in connection with the sacrifices concerning all the festivals other than Atzeret and Yom Ha-Kippurim? The Scripture did so in order to draw a comparison between Atzeret and Yom Ha-Kippurim. Just as Yom-Ha-Kippurim was a day of the giving of the Torah in that the second tablets were given on that day, so, too, was Yom Ha-Bikkurim the day on which the first tablets were given. In other words, the giving of the Torah occurred on Shavuot. But the Sadducees denied that the Torah was given on Atzeret, because they held that Atzeret always fell on a Sunday while everyone agrees that the Torah was given on the Sabbath. The Sages therefore deduced from the parallel construction "s'ir izim" confined to Shavuot and Yom Ha-Kippurim that those were the two days on which the Torah was given.
This is an extraordinary point over which I rejoiced as one who finds a treasure. And it would seem to me that the Rambam relied on this comparison between Yom Ha-Kippurim and Shavuot to support his opinion that the counting of the Omer in our days is Biblically mandated. Nevertheless, the Sages uprooted the name Shavuot to suggest through the name Atzeret the great and extraordinary event of the revelation of the Eternal on Mount Sinai on the day of Shavuot. The name "Atzeret," besides meaning the cessation of work, denotes a gathering together, as it is written in Joel 2:15, "call a solemn assembly" (qir'u atzeret). In the Torah as well, the day on which the Torah was given is referred to (Deuteronomy 9:10) as "yom ha-qahal." Thus, the name "Atzeret" corresponds exactly with name "yom ha-qahal," the day of the giving of the Torah.
To complete this discussion, I will mention an extraordinary matter which supports the view of the Rambam that the counting of sefirah is Biblically mandated in our days, which is the astonishing sight that the Torah divides (Leviticus 23:9-22) into separate sections the discussion of the bringing of the Omer (Id. 9-14), and the discussion of the counting of the Omer (Id. 15-22). And there must be a reason for this. But here is evidence that the masters of the tradition agreed with the view of the Rambam that the counting of the Omer is Biblically mandated even in our days and does not depend on the offering of the Omer. The Torah therefore discussed the commandment of the counting of the Omer separately from the offering of the Omer Counting is thus a separate commandment independent of the offering of the Omer and counting is practiced both when the Temple is extant and when it is not extant. And this evidence is ironclad.