Though sounding the shofar (ram's horn) on Rosh Hashanah is a biblical precept – no matter what day of the week the holiday may fall on – the Mishnah tells us that we do not sound the shofar when Rosh Hashanah falls on Shabbat:
If the holiday of Rosh Hashanah falls on Shabbat, the shofar is sounded in the Holy Temple, but not in the Land.
The Talmud gives us the explanation for this surprising law:
Rabbah said: All are under obligated to blow the shofar, but not all are skilled in the blowing of the shofar. Therefore, there is a danger that one will take the shofar and go to an expert to learn [how to properly sound it], and he will carry it four cubits in the public domain [—an act that is forbidden on the Shabbat].
Indeed, the Sages are empowered to "overrule" a Torah precept (if their instruction involves restraint from action, not a proactive violation of a biblical command). Our obligation to follow such directives is implicit in the verse, "And you shall do according to the word they tell you, from the place that G‑d will choose, and you shall observe to do according to all they instruct you."
In numerous chassidic discourses it is explained that it is inconceivable that the Sages would deprive all of Israel of the benefits afforded by one of the greatest mitzvot we have—simply on account of a few ignorant souls who might otherwise err. We must conclude that the Sages understood that on Shabbat it is actually unnecessary to blow the shofar, for that which we normally accomplish through sounding the shofar is, of its own accord, accomplished on Shabbat.
The accomplishment of the shofar is to renew G‑d's pleasure in His works, especially this world, so that there will be a desire to continue infusing His works with the life force necessary for their continued existence. If He delights in us, then He has reason to continue creating us, reason to continue His relationship with us. (For more on this, see The Kabbalistic Spin on Rosh Hashanah.)
The principal theme of Shabbat is also pleasure, delight and desire. "Call the Shabbat a delight," the prophet enjoins us—which we accomplish by partaking of sumptuous meals. And that which G‑d instructs us to do, He also does Himself. If so, on Shabbat the delight and desire to continue with our world and with our relationship is already there—no need to blow the shofar to renew it.
(Nevertheless, there are many gradations to pleasure. The level of Divine pleasure evoked through blowing the shofar in the Holy Temple – the location where G‑d's essence was manifest – is greater than the pleasure naturally activated on Shabbat. As such, the shofar is sounded in the Temple even when Rosh Hashanah falls on Shabbat.)
Soon, we hope to merit hearing the "great shofar" that will be sounded on the day of the Redemption: "And it shall come to pass on that day, that a great shofar shall be sounded, and those lost in the land of Assyria and those exiled in the land of Egypt shall come, and they shall prostrate themselves before G‑d on the holy mount in Jerusalem."
The "great shofar" symbolizes the ultimate level of pleasure, far greater even than the pleasure evoked in the Holy Temples of yore: the pleasure that G‑d takes in each and every one of His children. It is this pleasure that will be felt and manifest.
Indeed it is this immense revelation, this grand sounding of the shofar, that will reach the hearts of even the most distant of Jews – those lost in the land of Assyria and exiled in the land of Egypt – and kindle within them the desire to return to where they really belong: the holy mount in Jerusalem.
Wishing you and yours a sweet new year,
Rabbi Naftali Silberberg,
Chabad.org Editorial Team