Halakha Bilangan 10:9: Mishneh Torah, Fasts, Mishneh Torah, Shofar, Sukkah and Lulav & Sefer HaChinukh

It is a positive commandment from the Torah to cry out and to sound trumpets for all troubles that come upon the community; as it is stated (Numbers 10:9), "upon an enemy who attacks you and you sound trumpets." That is to say, [with] every matter that troubles you — such as famine, a plague, locusts and that which is similar to them — cry out about them, and sound the trumpets.

In the Temple on Rosh Hashanah, they would blow with one shofar and two trumpets [on its two] sides. The shofar would blow long and the trumpets would blow short, since the commandment of the day was with the shofar. And why would they blow it together with trumpets? Because it is stated (Psalms 98:6), "With trumpets and the blast of the shofar blow before the Lord, the King." But in other places on Rosh Hashanah, we only blow with a shofar exclusively.

The commandment of blowing the trumpets in the Temple and in war: To blow trumpets in the Temple each day as every sacrifice is offered, and also at a time of troubles, as it is stated (Numbers 10:9), “When you come to war, etc.” and it is written after it also (Numbers 10:10), “And on your joyous occasions, and your fixed festivals and new moon days, you shall blow the trumpets over your burnt-offerings and over the sacrifices of your peace-offerings, etc.” And even though the verse warns about a joyous occasion, a fixed festival and a new moon day, this is not precise. As they would blow with the trumpets over the sacrifice every day in the Temple. And so is it explained in Rosh HaShanah 29a: “Everyone is obligated in the blowing of the shofar — priests, Levites, and Israelites.” And the Talmud wonders about this in the Gemara, “Is this not obvious? [For] if they are not obligated, who would be obligated?” And it responds to it, “It was necessary [to say] priests, for it may enter your mind to say, ‘Since it is written (Numbers 29:1), “It is a day of blowing, etc.,” [you might have said that with regard to one who is obligated to sound only one day, he is obligated to sound the shofar on Rosh HaShanah.] But these priests, since they are [obligated] all year long, as it is written (Numbers 10:10), “you shall blow the trumpets over your burnt-offerings and over the sacrifices of your peace-offerings, etc.” [you might say that they are not obligated].’” The Talmud then asks, “Are they similar? There it is a trumpet and here it is a shofar!” Nevertheless, we learned [from this] that throughout the whole year — meaning to say on each and every day — there were trumpets in the Temple. And they said in Tractate Arakhin 13a that we do not reduce to less than twenty-one blasts in the Temple and we do not increase to over forty-eight.