Although Chazal instituted birkas Shehechiyanu before performing mitzvos which come around only periodically, such as netilas lulav and ner Chanukah, Sefiras HaOmer is an exception.
Some say that we don’t make a Shehechiyanu on Sefiras HaOmer because that brachah is reserved for times of joy and pleasure, while Sefirah coincides with mourning over the deaths of the talmidim of Rabi Akiva (Chok Yaakov 489:7, quoted in Taamei Haminhagim). Also, the mitzvah of Sefirah is a remembrance of the destruction of the Beis Hamikdash; accordingly, we don’t listen to music during this period, and following each day’s counting we say a tefillah for the rebuilding of the Mikdash (Matteh Moshe 667).
Others explain that since Sefiras HaOmer is a counting toward and preparation for Shavuos, the brachah is made then, at the culmination of the mitzvah (Pachad Yitzchak 70, citing Radbaz). Even when the Beis Hamikdash stood, Sefiras HaOmer was designed to remind those who were scattered about the land and occupied with harvesting and other activities that the time was approaching to return to Yerushalayim for the Yom Tov of Shavuos (Avudraham).
Preparing for Kabbalas haTorah is not, of course, just a matter of keeping in mind how many days are left before making reservations at the magnificent new Hotel Shmurah and hitching up the wagon for the trip to the Mikdash. We have to lay a foundation for receiving the Torah.
It is no accident that the dispute between Chazal and the Tzedokim centered on the right time for harvesting the omer, which is the beginning of the path to Mattan Torah on Shavuos. Chazal gave emphasis to the dispute with the heretics of their time, including a thorough discussion of it in the Talmud (Menachos 65a). For the Tzedokim, this particular issue was a linchpin of their doctrine denying the authenticity of Torah Sheb’al Peh. Chazal set out to demolish the heresy, which they did with a series of irrefutable proofs.
The Tzedokim insisted on a literal interpretation of the Torah passuk, saying that “the day after this Shabbos he shall wave it” means the day after the weekly Shabbos; whereas Chazal taught that the word Shabbos refers here to Yom Tov (the first day of Pesach, or 15 Nisan), and the day for cutting the Omer was the next day, 16 Nisan.
If the Torah meant that the korban omer was to be brought after the weekly Shabbos some time after Yom Tov, as the Tzedokim contended, we would have no way of knowing which Shabbos in the year was meant.
In addition, the word Shabbos in the passuk about counting — “until the day after the seventh Shabbos” (23:26) — must also refer to the weekly Shabbos according to the Tzedokim, and therefore the 50th day (Shavuos) would be not be “the day after the seventh Shabbos,” but the day after the eighth Shabbos. (Harav S.R. Hirsch, Vayikra 23:11)
To drive the point home, the Chachamim transformed the cutting of the barley into a public refutation of the Tzedokim. On Motzoei Yom Tov, as they were about to cut the barley, they would ask, “This Shabbos?” (meaning now, the day after this Yom Tov), and the assemblage would respond, “Yes!” (Mishnah, Menachos 10:3) This was repeated again and yet again for good measure, and they would raise their voices, so that the heretics would hear it, to “remove it [the heresy] from their hearts.” (Commentary of Rav Ovadia of Bartenura)
From all this we can learn that the first step in the preparation for Mattan Torah entails an acceptance of Torah Sheb’al Peh, the Oral Law handed down to us generation after generation by our Chachamim.
But proper preparation for receiving the Torah entails not only an authentic philosophy; it needs personal purification as well.
Chazal say that we should stand when counting the Omer, comparing us to barley standing in the field (likening the word komah, standing, to kamah, the standing grain). We are to think of ourselves as if we too are being severed from the earth. Just as the reaping of the barley, cutting it from its roots in the earth, prepares it to be offered on the altar, so too we are to sever our attachment to the earth, to mere material existence. Each night that we count toward the Yom Tov of Mattan Torah, we exert ourselves to become less and less connected to the earth; more and more to the spirituality of Torah. (Harav Shimon Schwab zt”l, Maayan Bais Hashoevah, pp. 219, 285)
In Tehillim 118, we read Isru Chag ba’avosim ad karnos hamizbei’ach, bind the festival to the horns of the altar. The Talmud (Sukkah 45b) interprets this passuk as kol ha’oseh issur l’chag b’achilah u’shsiah, if on the festival one holds back from eating and drinking, it is as if he has built a mizbei’ach and brought a korban on it.
Maharsha explains that on Yom Tov one should eat and drink only l’shem Shamayim, refraining from excess. The food and drink that one does not indulge in is compared to a sacrificial offering to Hashem.
In fact, our table all during the year is compared to the Mizbei’ach — if we eat and drink l’shem Shamayim.
With that in mind we begin the approach to Shavuos, elevating ourselves day by day through our dedication to serving Hashem.