The Torah refers to two days as "atzeres": the eighth day of Sukkos (Vayikra 23:36, Bemidbar 29:35) and the seventh day of Pesach (Devarim 16:8). Chazal add a third day to the list - Shavuos (see Megilla 3:5, Rosh Hashana 1:2 and many other places). The mystery of this unique terminology deepens when we consider the fact that Shavuos traditionally celebrates the day of Mattan Torah, and the seventh day of Pesach commemorates the miraculous splitting of the Sea. But nowhere does the Torah explicitly link these two holidays with the aforementioned crucial events in Jewish history. Why doesn't the Torah explicitly make these links, and what do all these days of atzeres have in common?
On a simple plain, the Biblical name atzeres which literally means "held back" or "gathering" (see Ibn Ezra and Seforno to Vayikra ibid.) appropriately applies to the last day of a Yom Tov since we "gather" together one last time in our Batei Keneisiyos and "hold back" from engaging the world and we "hold back" the holiday for a while before it leaves us. Ramban (ibid.) famously views Shavuos as, in a sense, the last day of Pesach with the Sefiras Ha'Omer period serving as a type of Chol Hamo'eid linking Pesach and Shavuos. Since the entire goal of the Exodus - commemorated and relived on Pesach - was the receiving of the Torah celebrated on Shavuos, the two holidays are integrally linked.
Perhaps we can suggest another insight into this title. Of all the holidays in the Jewish calendar, only those called atzeres have no unique mitzvos associated with them other than the prohibition of melacha. The first night of Pesach overflows with distinctive mitzvos; the first day of Sukkos is celebrated with both Sukka and the four species. Rosh Hashana is characterized by its shofar, Yom Kippur through its fasting and repentance. But the atzeres of Pesach and of Sukkos, and the atzeres of Shavuos which Chazal, in their wisdom, understood shares a common quality with the other atzaros, have no unique positive commandments.[1] It may be that Rashi (Bemidbar ibid.) alludes to this by writing that atzeres means "held back from work". But all the Yamim Tovim are "held back from work"; why should only the ones listed above qualify to be called atzeres? We can suggest that the other holidays have additional commandments associated with them; the term atzeres is reserved for those days on which are commanded only concerning work.
But the question still remains as to why the Torah does not command any specific mitzvos to be observed on these holidays? In answer to this question, perhaps we can suggest as follows. Many have expounded upon the interrelationship of the dual thrusts of avodas Hashem: the maasei hamitzvos and the all encompassing spiritual, emotional bond between created and Creator which they are meant to evince and which they represent. To be sure, as Rav Yosef Dov Soloveitchik zt"l adjured, the "love gesture" is secondary, even if crucial, not primary: "first the mitzvah act, then the great romance!" Many reasons can be presented as to why there is such stress on the "mitzvah act". Pesach certainly stresses this idea with the oft-repeated verse "בעבור זה עשה ד' לי בצאתי ממצרים - because of this Hashem acted for me when I left Egypt" (Shemos 13:8). Mori v'Rabi Rav Hershel Schachter shlita and others (see Beis HaLeivi, Bo, ibid. s.v. "v'higad'ta livincha") often stressed that Chazal interpret "בעבור זה" as pointing to something, namely referencing the mitzvah objects of the Seder. Hence, the verse should be read, "because of this", namely, in order to fulfill His commandments, "G-d took me out of Egypt"; the whole purpose of the Exodus was to fulfill His commandments.
But the deveikus, the cleaving to G-d, that these mitzvos are meant to produce must be stressed as well. Sifrei Chasidus teach that the very word mitzvah is etymologically related also to the Aramaic word, tzavta, company or communion. Sefas Emes even notes that the enormous yearning for Hashem the soul feels for its Source when performing a mitzvah is so intense that it is only by specific Divine Will that the soul is able to remain in the body. By following the Divine commandments, we do no less than connect to the Infinite One Himself.[2] Ignoring this crucial component of Divine service leads to the rebuke of the prophets (Yeshayahu 29:13) that we fulfill the Divine commandments as "מצות אנשים מלומדה" - rote performance of raw acts devoid of any meaning or feeling, or, using Rav Soloveitchik's terminology, missing the "great romance". A famous chassidic anecdote told about the first Lubavitcher Rebbe, Rav Schneur Zalman of Liadi, relates that once some chassidim asked their Rebbe if they should convince their father to become a chasid. Upon which the Rebbe inquired, "Does your father do mitzvos with hislahavus (fervor)?" The sons responded, "Tatte kisses the schach of the sukka when he thinks we aren't watching!" "If so," responded the Rebbe, "you can leave him be." Attached to Yamim Tovim replete with almost frenetic mitzvah activity leading up to and on the early part of the festivals, we also have days of atzeres, days on which we still do not engage the world but without specific mitzvos. These are days of being alone with Hashem, days of reflecting the great spiritual adventure begun at the Exodus, continued in the midbar, brought to a crescendo at har Sinai and in Eretz Yisrael, continued with an eternal - even if at sometimes low - flame throughout the long exile, awaiting the return of the zenith of the revelation of the Divine presence in the Messianic era. These are days of lev and nefesh, not packed with ma'asei hamitzva, but of reflection of the "dizzying heights" to which all the ma'asei hamitzva are meant to direct us.
Perhaps this is also why the splitting of the Sea is not explicitly linked to the last day of Pesach. As Chazal inform us, keriyas Yam Suf was so exalted that even the maidservant reached the highest level of prophecy.[3] All of Bnei Yisrael exclaimed, "זה קלי ואנוהו" - as if pointing to Hashem himself. The same of course is true of the day of Mattan Torah, the event in which "Heaven and Earth kissed." Had the Torah explicitly mentioned these events, it would have, to some extent, implied that these events can be described in words, in regular worldly terms. But they were mysterious, even transcendent, reflecting paradoxical communion of seemingly lowly, finite creations with their Infinite Creator. These days of the seventh day of Pesach and of Shavuos, both representing the pinnacle of the Exodus, are too lofty to be described ordinarily. We know intellectually the events they commemorate, but we celebrate holistically the intense connection to the Divine they reflect.
May Hashem always grant us and assist us in generating the will and ability necessary to live these two crucial components of Torah observance.
[1]The view made famous by Vilna Gaon that there is a mitzva kiyumis of eating matza on all of Pesach seems to go against this proposal. But firstly, not all agree with this view; secondly, some interpret his view as being an active indication of the negative commandment against consuming chametz; thirdly, even Vilna Gaon concedes it is not obligatory.
[2]See end of The Mystery of the Ketores for one small presentation of these crucial concepts.
[3]See also Guide to the Piscatorially Perplexed. The themes developed there concerning water and sealife may be applied here as well.